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BASIC LABORATORY TECHNIQUES (Revised 1-2-16) (See Appendix II: Summary for making Spreadsheets and Graphs with Excel and Appendix III parts C, C1 and C2: Significant figures, scientific notation and rounding)

A. WEIGHING

The determination of the quantity of matter in a sample is most directly determined by measuring its mass. The process by which we determine the mass of an object is commonly referred to, incorrectly,

as weighing. There are two closely related quantities, weight and mass. The weight of an object is the force which the earth's gravitational field exerts on the object. This force diminishes as the object is

moved to a greater distance from the center of the earth (e.g., the weight of an object is actually less on the top of a mountain than at sea level.) The weight of an object, therefore, depends upon gravity and consequently upon the object's location. This can be expressed mathematically as:

w = mg

where w = the force of attraction of the earth for the object, or weight m = mass of the object

g = gravitational acceleration (varies with distance from the earth’s center)

In this equation we have defined mass. Mass is a measure of quantity of matter and does not depend on other variables as does weight. In carrying out quantitative experiments, we wish to determine the mass rather than the weight of samples. This can be accomplished using a set of objects of accurately known

mass—these are called weights. We compare the weight (force of attraction) of our unknown mass to the weight of the known masses. When these two forces are equal, they have the same mass.

1. Laboratory Balances

We will carry out mass measurements on electronic balances that rapidly compare the mass of an object placed on the top pan with calibrated mass objects inside the instrument and read out the mass in

digital form. Since balances are precise and delicate instruments, every precaution to prevent damage must be

taken. The following rules are necessary if the balances are to remain accurate scientific instruments. a) Never move the balances. Some contain knife-edges which may be damaged if the balance is

jarred. To work properly a balance must be level. Moving the balance may change the level. b) Never place any chemicals directly on the balance pan. Place materials to be weighed on

weighing paper or in a suitable container.

c) If your balance isn’t working properly, do not try to fix it. Call the instructor. d) If anything is spilled on the balance (solid or liquid) remove it immediately.

e) Never weigh anything hot on the balances. There is the possibility of damage to the balance, as well as the likelihood of weighing errors due to buoyancy effects from the warmer air surrounding the object.

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There are simple-to-use electronic, digital balances in the laboratory. The balances require a 30 minute warm-up and therefore will be left on continuously. The maximum amount that can be

weighed on these balances is either 110 or 150 grams, depending on which type of balance you are using. When that weight has been exceeded the readout will be "H."

These balances can weigh in different modes. You will be using only the grams mode, which

has the following symbol on the readout: "g." If the readout is different see your instructor!!

All of these balances will give you the mass to the nearest milligram, or 0.001 g. Make sure

that you record every mass to three digits after the decimal point! That is, you should always have numbers (even if they are zeros) in the tenths, hundredths, and thousands of a gram places. The unit for all of your masses should be grams, not milligrams, however.

2. Direct Weighing

a) Place either a piece of weighing paper or weighing container (lightweight plastic beaker) on the balance pan and tare or zero the balance by pressing "TARE" on the front (left or right side) of the

balance. All 0’s will appear on the readout, indicating that the mass of the container has been subtracted.

b) Now place the material to be weighed on the paper or into the weighing container and the readout will be the mass in grams. Remember to record three digits after the decimal point. If the

balance does not show three places there, see your instructor. 3. Weighing by Difference

a) Make sure balance is in grams mode. b) Press the TARE key to zero the balance with nothing on the balance pan.

c) Weigh the container and record its mass. d) Add the material to the container and reweigh.

e) Find the mass of material alone by subtracting the two weights. f) Use the same balance for the two weighings.

B. USING VOLUMETRIC GLASSWARE

Most of the glassware in your laboratory locker has been marked by the manufacturer to indicate the volume of liquid contained by the glassware when filled to a certain level. The graduations etched

or painted onto the glassware by the manufacturer differ greatly in the precision/accuracy they indicate, depending on the type of glassware and its intended use. For example, beakers and Erlenmeyer flasks are marked with very approximate volumes, which serve merely as a rough guide to

the volume of liquid in the container. Other pieces of glassware, notably burets, pipets, and graduated cylinders, are marked much more precisely by the manufacturer to indicate volumes. It is important to

know when a precise volume determination is necessary and appropriate for an experiment, and when only a rough determination of volume is needed.

Glassware that is intended to contain or to deliver specific precise volumes is generally marked by the manufacturer with the letters "TC" (to contain) or "TD" (to deliver). For example, a flask that

has been calibrated by the manufacturer to contain exactly 500 mL of liquid at 20 oC would have the legend "TC 20 oC 500 mL" stamped on the flask. A pipet that is intended to deliver a 10.00 mL sample of liquid at 20 oC would be stamped with "TD 20 oC 10 mL." It is important not to confuse

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"TC" and "TD" glassware. Such glassware may not be used interchangeably. The temperature (usually 20 oC) is specified with volumetric glassware since the volume of a liquid changes with

temperature, which causes the density of the liquid to change. While a given pipet will contain or deliver the same volume at any temperature, the amount of substance present in that volume will vary

with temperature. 1. Graduated Cylinders

The most common apparatus for routine determination of liquid volumes is the graduated cylinder. Although a graduated cylinder does not permit as precise a determination of volume as do

other volumetric devices, for many applications the precision/accuracy of the graduated cylinder is sufficient. Examine the graduated cylinders in your lab locker and determine the smallest graduation

of volume that can be determined with each cylinder.

When water (or an aqueous solution) is contained in a narrow glass container, such as a

graduated cylinder, the liquid surface is not flat as might be expected. Rather, the liquid surface curves upward where it meets the container walls (see figure below). This curved surface is called a

meniscus, and is caused by an interaction between the water molecules and the molecules of the glass container wall. When reading the volume of a liquid that makes a meniscus, hold the cylinder so the meniscus is at eye level, and read the liquid level at the bottom of the curved surface.

Reading a meniscus. Read the bottom of the meniscus while

holding at eye level. Always record the volumes of 100 mL graduated cylinder to +/- 0.1 mL (one place after the decimal).

2. Pipets

When a more precise determination of liquid volume is needed than can be provided by a graduated cylinder, a pipet may be used. Pipets are especially useful if

several measurements of the same volume are needed (such as in preparing similar-sized samples of a liquid unknown). Two types of pipet are commonly available. The Mohr pipet is calibrated at each milliliter and can be used to deliver any size sample (up to the capacity of the pipet). The volumetric

pipet can deliver only one size sample (as stamped on the barrel of the pipet), but

generally it is easier to use and is more reproducible. Always record the volume of a pipet to +/- 0.01 mL (two places after

the decimal).

Pipets are filled using a rubber bulb to supply the suction needed to draw liquid into the pipet. IT IS ABSOLUTELY FORBIDDEN TO

PIPET BY MOUTH IN CHEMISTRY LAB!!!

The safety bulb should not actually be placed around the barrel of the pipet. This would most

likely cause the liquid being measured to be sucked into the bulb. Rather, squeeze the bulb, and merely press the opening of the bulb against the opening in the barrel of the pipet to apply the suction force, keeping the tip of the pipet under the surface of the liquid being sampled. Your

instructor will show the class how to use a pipet at the start of the period.

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Here is a description of the technique: Allow the suction to draw liquid into the pipet until the liquid

level is 1 or 2 inches above the calibration mark of the barrel of the pipet. At this point, quickly place your index finger over the opening at the top of the pipet to prevent the liquid level from

falling. By gently releasing the pressure of your index finger from the pipet opening, the liquid level can be allowed to fall until it

reaches the calibration mark of the pipet. The tip of the pipet may then be inserted into the container which is to receive the sample, and the pressure of the finger removed to allow the liquid to flow

from the pipet.

When using a pipet, observe the following rules: a) The pipet must be scrupulously clean before use: wash with soap and water, rinse with tap

water, followed by distilled water. If the pipet is clean enough for use, water will not bead up

anywhere on the inside of the barrel. b) To remove rinse water from the pipet (which would dilute the solution to be measured), rinse

the pipet with several small portions of the solution to be measured, discarding the rinsings in a waste beaker for disposal.

c) The tip of the pipet must be kept under the surface of the liquid being drawn up during the

entire time suction is being applied, or air will be sucked into the pipet. d) Allow sufficient time for the pipet to drain when emptying, to make certain the full capacity

of the pipet has been delivered. Remove any droplets of liquid adhering to the tip of the pipet by touching the tip of the pipet to the side of the vessel that is receiving the sample. Do not shake.

e) If you are using the same pipet to measure out several different liquids, you should rinse the

pipet with distilled water between liquids, and follow with a rinse of several small portions of the next liquid to be measured.

C. RECORDING DATA CORRECTLY USING SIGNIFICANT FIGURES

Many operations in the chemistry laboratory involve measurements of some kind. Examples are weighing a compound or measuring the volume of a liquid. It is important to record these data properly so that the number recorded correctly represents the certainty of the measurement. The

following discussion is intended to serve as a guide for calculating and reporting numerical results.

Every measurement that is made is really an approximation. For example, the length of the object below is between 1.5 and 1.6 units. Its length is seen to be approximately 1.56 units. There is uncertainty in the last digit, 6; it is estimated. If you had an object with the recorded weight of 2.6

grams, this would mean that the object was weighed to the nearest tenth (0.1) of a gram and that its exact weight was between 2.5 g

and 2.7 g. In recording a result, it is the last digit that represents a degree of uncertainty; for example, in 2.6 g it is the number 6 that represents some degree of uncertainty. We say the number 2.6 g

contains two significant figures, the numbers 2 and 6 being the significant figures. If the recorded weight of the

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object were 2.634 g, there would be four significant figures (2, 6, 3, and 4), and this would mean that

the object was weighed to the nearest thousandth (0.001 g). [Note: The 2 is in the ones place, the 6 in the tenths place, the 3 in the hundredths place and the 4 in the thousandths place.] Thus, it is the 4

(bolded) that has been estimated. Significant figures refer to those digits we know with certainty plus the first doubtful or estimated digit.

It is important that you be aware that every measuring device, regardless of what it may be, has limitations in its accuracy. Moreover, to take full advantage of a given measuring instrument, you

should be familiar with or evaluate its accuracy. Careful examination of the subdivisions on the device will indicate the maximum accuracy which you can expect of that particular tool. In this experiment you will determine the accuracy of your 10 mL pipet, 10 mL graduated cylinder, and a

device called a RePipet. This device is useful for delivering a fixed volume of liquid relatively quickly. The approximate accuracy of some of the equipment you will use in CHEM 109 is in Table 1

below. Not only should you obtain a measurement to the highest degree of accuracy that the device or

instrument permits, but you should also record the reading or measurement in a manner that reflects the accuracy of the instrument. For example, a mass obtained from a “top loader” balance should be

observed and recorded to the nearest 0.001 g. The final digit of each “uncertainty” in Table 1 represents the estimated or uncertain digits and must be recorded. Table 2 gives examples of correctly and incorrectly recorded data for the thermometer and top-loading balance.

D. EVALUATING EXPERIMENTAL DATA

If any piece of experimental data is to be of much use, some idea of the reliability of its values is important. The measurement of any physical quantity is subject to some degree of

uncertainty, depending on the equipment or instruments used to make the measurement as well as on the skill of the experimenter. There are two components to this uncertainty: accuracy and precision.

Precision refers to how closely several determinations of one quantity agree with each

other. In other words, are measurements made on the same sample under the same conditions

relatively close together (high precision), or spread out (low precision?) Precision can be expressed in terms of "deviation," the absolute value of the difference between each measured value and the mean or average of that set of measurements. Taking the absolute value of a number means making the

number positive. Therefore the deviation will always be a positive number.

Table 1. Equipment Accuracy

Equipment Uncertainty

top loading balance +/-0.001 g (+/-1 mg)

meter stick +/-0 .01 cm (+/-0.1 mm)

thermometer +/-0.2 oC

10-mL graduated cylinder +/-0.01 mL

100-mL graduated cylinder +/-0.1 mL

10-mL volumetric pipet +/-0.02 mL

50-mL buret +/-0.02 mL

Table 2. Correct and Incorrect Data

Thermometer Top Loading Balance

86 oC (incorrect) 85.9 g (incorrect)

85.9 oC (correct) 85.93 g (incorrect)

85.93 oC (incorrect) 85.932 g (correct)

85.932 oC (incorrect) 86 g (incorrect)

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Example of calculating deviation: For the following set of numbers (1.452, 1.211, 1.389 and 1.198) whose average is 1.313, the deviation of each number would be found by subtracting that number from

the average (then taking the absolute value ̧ ie. making it positive.) |1.452 − 1.313 | = 0.139 |1.389 − 1.313 | = 0.076 |1.211 − 1.313| = 0.102 |1.198 − 1.313| = 0.115

Accuracy refers to how closely a measured value approaches the true or accepted value. It

can be expressed experimentally by the percentage of error:

(Experimental Value - True Value) x 100

% Error = True Value Note that the % error may be negative or positive, and that this sign (+ or −) clearly has information content.

Measurements showing a high degree of precision do not always show a high degree of accuracy and vice versa. See figure below.

w = good precision, poor accuracy x = poor precision, poor accuracy

y = poor precision, good accuracy? (Find the mean [center] of these points to understand.) z = good precision, good accuracy

Your BLT report includes: (1) Pages 31-33 (2) Your spreadsheet. (Remember to include row and column headings on your spreadsheet and formula page.)

(3) Formula page. (To view the formulas, press “Ctrl” and “~” simultaneously.

Note: The pre-lab, which is due at the beginning of the recitation, is located on page 29.

w

ww

x

x

x

y y

y

zz

z

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PROCEDURE: A. Significant figures and measurements

Follow the instructions for part 1. For part 2a, make sure you measure the length in cm (not inches). If your ruler is labeled in mm, you must convert to cm. For part 2b, measure the temperatures in oC. To make an ice bath, fill a beaker with ice from the ice machine. Then add water and swirl the beaker to cool the water. Let it sit for a couple of minutes with the thermometer resting inside, then swirl the beaker again before measuring the temperature. To record the temperature of tap water, fill a 400 mL beaker with tap water and let the thermometer sit in the water for several minutes before making the reading. To correct your room temperature and tap water temperature readings, determine how far your ice-water reading deviate from 0.0 oC (the true value for the temperature of ice-water.) This is a measure of how far “off” your thermometer was. Subtract/add this deviation from/to your thermometer readings to get the corrected temperature values.

B. Laboratory Balances

You are to weigh one of the given objects by the “direct method” on two different balances. (See pp. 21-22 for instructions.) Record the number of each balance you use on the data sheet. You are then to weigh the same object on one of the balances using the “weighing by difference” technique. Record weights on the data sheet to three places after the decimal! Small plastic beakers are to be used as weighing containers.

C. Reading a Graduated Cylinder

Your instructor will set up a display of graduated cylinders filled with different amounts of water. Read the volume of liquid contained in a 10 mL and 100 mL cylinder. (See p. 23) Record your readings using the correct number of significant figures after the decimal point as permitted by the precision of each cylinder.

D. Evaluation of the Precision and Accuracy of various volumetric measuring devices 1. The Graduated Cylinder. This exercise will demonstrate the accuracy with which a 10 mL graduated cylinder is able to deliver 10 mL of water. You will be recording your data on page 31. Clean and dry a 50 mL plastic beaker and weigh to the nearest milligram (three places after the decimal point) once. Record the same weight on the data sheet (page 31) for all three trials. Use the 10 mL graduated cylinder to transfer 10 mL of water to the beaker. Reweigh the beaker containing the water and subtract to determine the mass of water transferred by the graduated cylinder. Do three trials, in total, thoroughly drying the beaker after each trial.

Enter your data on the copy of the spreadsheet on page 28 and on the spreadsheet that you can download from http://faculty.uscupstate.edu/jkrueger. Enter appropriate formulas to perform the calculations. Do not put

units on the spreadsheet! Refer to “Summary for Making & Printing Spreadsheets & Graphs” in Appendix II in your lab manual. You will need to determine the mass of the water transferred, and then calculate the average value for the mass of water transferred. Calculate the deviation of each individual measurement from the average by subtracting each measurement from the average. Your instructor will give you the true value for the mass of 10 mL of water at the current room temperature. Record the value on your data sheet and use it to calculate the percent error for your three measurements of the mass of water. Find your average % error. Use correct sig. fig. On page 31, you also need to show three hand calculations for trial 1. Remember to use correct sig. fig and units!

2. The Pipet. This procedure demonstrates the accuracy of a pipet in delivering 10 mL of water. Clean and dry a 50 mL plastic beaker and weigh to the nearest milligram. Record the weight on your data sheet. Half fill an Erlenmeyer flask with water. Pipet 10 mL of water from the flask into the beaker. Reweigh the beaker and the water. Enter your data on page 31 and again on the spreadsheet. You will again need to use appropriate formulas to perform the calculations. Check that you have correct sig. fig. for each answer. 3. The RePipet II. Repeat as above in #2, but using the RePipet II pipetting device. Your instructor will demonstrate its use. It will be pre-set, so you should not adjust the knob on the plunger. Record your data on page 31, and then on the spreadsheet. Use formulas to determine the values asked for. Sign your spreadsheet in the box to indicate your agreement with the Honor Code statement. Print a formula page for your spreadsheet. To show formulas, press the “Ctrl” button and “~” buttons simultaneously.

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Questions that may be discussed in recitation.

1. How will you determine if your thermometer is reading accurate?

2. For the following 3 measurements for the mass of ten mL of water: 9.981 mL, 10.014 mL, and 9.799 mL, (a) calculate the following and (b) write a formula for these calculations that could be

entered in a spreadsheet as shown below. Check out Appendix II, to help you with part (b).

a) The average

b) The deviation of the measurement in B3.

c) The % error for the measurement in B3 if the true value is 9.975 mL.

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BASIC LABORATORY TECHNIQUES Sec: Name:

Pre-Lab Questions Date:

1. a) Indicate the place (e.g. ones, tens, tenths) for each of the digit in the measurement 43.906. 4___________ 3____________ 9 ___________ 0____________ 6____________

b) Which digit is most likely to be in error in this number? ____________

2. What does “weighing by difference” mean?

3. Explain how a pipet bulb is used to measure liquid with a pipet.

4. Why does a meniscus form on the surface of water or an aqueous solution?

5. Should you add units to the numbers on a spreadsheet? _________

6. How many digits to the right of the decimal point should you record when using the lab

balance?___ 7. If a thermometer is accurate, what temperature should it read in oC when placed in ice water?

_____

8. (True/False) Beakers and Erlenmeyer flasks are accurately marked with volume measurements

and can be used for precise measuring.

9. You weigh a sample of about 170 g on the lab balance and it displays an H instead of the

weight. What is going on?

10. Refer to the equipment list at the end of the safety information. Draw a sketch of a beaker

and of an Erlenmeyer flask. Label each drawing.

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Data and Results Sheet Sec: Name: A. (0.13) Significant figures and measurement Date:

1. Circle the correctly recorded measurement (sig. fig.) for each piece of lab equipment.

a. Meter stick (or metric ruler) 46.34 cm 46.3 cm 46 cm

b. Pipet (volumetric) 10.00 mL 10.0 mL 10. mL

c. Thermometer 79. o C 79.2 o C 79.22 o C

d. Graduated cylinder (100 mL) 19 mL 19.3 mL 19.31 mL

e. Top-loader balance 56.9 g 56.92 g 56.924 g

2. Record the following measurements:

a. Metric ruler: Length of this page Width of this page

Area of this page

b. Temperature in oC: Temp. of ice water

Room temp. Corrected room temp. (See near top of page 27.)

Tap water temp. Corrected tap water temp. B. (0.14) Weighing Material Code #

1. Direct Weighing:

1st Balance mass of material

2nd Balancer mass of material

Difference in masses between the two balances

2. Weighing by Difference:

Balance mass of beaker and material

mass of empty beaker

mass of material

C. (0.02) Reading a Graduated Cylinder – read both a 100 mL and a 10 mL cylinder

Volume in cylinder

Volume in cylinder

(Sp 2017) Instructor Initials ______

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D.(0.09) Fill columns C and D with appropriate data. Fill in unshaded cells in columns E, F and G. Show calculations for trial 1 below. On the spreadsheet, you must use equations for the calculations in E - G.

Show set-up and work for the following three calculations

(0.03) Show calculation for mass of water (0.04) Show calculation for deviation

for Trial 1 for Grad. Cylinder: from the mean for Trial 1 for Grad.

Cylinder

(0.04) Show calculation for % error for Trial 1 for Grad. Cylinder:

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Post lab Questions:

1.(0.03) Complete the Summary Table from your spreadsheet: Average Mass of Water Average Deviation Average % error

Graduated

cylinder

Pipet

RePipet

2. (0.04) Based on your data, which device (graduated cylinder, 10 mL pipet, or the Repipet) is MOST precise in its delivery? Explain how you reached this conclusion, referring to your average deviations for all three. (This explanation is required for full credit.)

3. (0.04) Based on your results, which device (graduated cylinder, 10 mL pipet, or the Repipet) is MOST accurate in its delivery? Explain. (What numbers are important for this determination?)

4a)(0.01) If you were asked to dispense 10. mL (+/- 1. mL), what would be the smallest and largest volumes that would be acceptable. smallest________________ largest ___________________ b)(0.01)If you had to deliver 10. mL (+/- 1. mL) to each of 250 different beakers, then which of the three devices (graduated cylinder, pipet or Repipet) would you use? Briefly explain your answer stating what factors affected your decision.