Measurement Notes

Chapter 2

LengthScientist use the metric system—a standard measurement

system based on the #10.The meter is the basic unit.millimeter = mm King Kilo(100 centi) centimeter = cm Henry Hecto(10 deci) decimeter = dm Died Deca meter = m While Whole(1,000 kilo) kilometer = km Drinking Deci10 mm = 1 cm Chocolate Centi10 cm = 1 dm Milk Milli100 cm = 1m1000 m = 1 km

Metric UnitsMass refers to the amount of matter in an object.

The base unit of mass in the metric system in the gram and is represented by g.

Standard: 1 kilogram is equal to the mass of the International Prototype Kilogram (IPK), a platinum-iridium cylinder kept by the BIPM at Sèvres, France.

Metric Units

1 Kilogram (kg) = 1000 Grams (g)

1 Gram (g) = 1000 Milligrams (mg)

Which is larger?A. 1 kilogram or 1500 grams

B. 1200 milligrams or 1 gram

C. 12 milligrams or 12 kilograms

D. 4 kilograms or 4500 grams

Kilogram Prototype

Balance Rules In order to protect the balances and ensure accurate results, a number of rules should be followed:

Always check that the balance is level and zeroed before using it. Do not weigh hot or cold objects. Clean up any spills around the balance immediately.

Measuring Mass

Top Image: http://www.southwestscales.com/Ohaus_Triple_Beam_750-SO.jpgBottom Image: http://www.regentsprep.org/Regents/biology/units/laboratory/graphics/triplebeambalance.jpg

We will be using triple-beam balances to find the mass of various objects.

The objects are placed on the scale and then you move the weights on the beams until you get the lines on the right-side of the scale to match up.

Once you have balanced the scale, you add up the amounts on each beam to find the total mass.

What would be the mass of the object measured in the picture?

_______ + ______ + _______ = ________ g

Measuring Mass – Triple-Beam Balance

1st – Place the film canister on the scale.

2nd – Slide the large weight to the right until the arm drops below the line. Move the rider back one groove. Make sure it “locks” into place.

3rd – Repeat this process with the top weight. When the arm moves below the line, back it up one groove.

4th – Slide the small weight on the front beam until the lines match up.

5th – Add the amounts on each beam to find the total mass to the nearest tenth of a gram.

Check to see that the balance scale is at zero

Volume

Volume is the amount of space a substance occupiesThe liter (L) is the base unitTools: metric ruler is used to measure regular solids v=l x w x h (cm3) or a graduated cylinder for liquids and irregular solids (mL)Meniscus- the curved surface of liquid resulting from

surface tension

Graduated Cylinders

The glass cylinder has etched marks to indicate volumes, a pouring lip, and quite often, a plastic bumper to prevent breakage.

Reading the Meniscus

Always read volume from the bottom of the meniscus. The meniscus is the curved surface of a liquid in a narrow cylindrical container.

Try to avoid parallax errors. Parallax errors arise when a meniscus or needle is viewed from an angle rather than from straight-on at eye level.

Correct: Viewing the meniscus

at eye level

Incorrect: viewing the meniscus

from an angle

Significant Figures Significant figures in measurement include the all certain digits and one uncertain digit. Significant figures communicate how precise measurements are.

Certain digits are determined from the calibration marks on the cylinder.

The uncertain digit (the last digit of the reading) is estimated.

Use the graduations to find all certain digits

There are two unlabeled graduations below the meniscus, and each graduation represents 1 mL, so the certain digits of the reading are…

52 mL.

Estimate the uncertain digit and take a reading

The meniscus is about eight tenths of the way to the next graduation, so the final digit in the reading is

The volume in the graduated cylinder is

0.8 mL

52.8 mL.

Self TestExamine the meniscus below and determine the volume of liquid contained in the graduated cylinder.

The cylinder contains:

_ _ . _ mL7 6 0

Irregular Volume

A solid with an irregular volume will be measured using the displacement method

1. Measure and record an amount of water2. Drop the irregular object in the cylinder3. Read the cylinder again and record the

amount4. Subtract the two measurements and

the result is the volume of the irregular object in ml

The Thermometero Determine the temperature by reading the scale on the thermometer at eye level. o Read the temperature by using all certain digits and one uncertain digit.

o Certain digits are determined from the calibration marks on the thermometer. o The uncertain digit (the last digit of the reading) is estimated. o On most thermometers encountered in a general chemistry lab, the tenths place is the uncertain digit.

Do not allow the tip to touch the walls or the bottom of the flask.

If the thermometer bulb touches the flask, the temperature of the glass will be measured instead of the temperature of the solution. Readings may be incorrect, particularly if the flask is on a hotplate or in an ice bath.

Reading the ThermometerDetermine the readings as shown below on Celsius thermometers:

_ _ . _ C _ _ . _ C 8 7 4 3 5 0

Mass

• Mass is the amount of matter in an object

• It is determined by using a balance• The unit for mass is grams.

English vs. Metric Units

Which is larger?

1. 1 Pound or 100 Grams

2. 1 Kilogram or 1 Pound

3. 1 Ounce or 1000 Milligrams

1 pound = 453.6 grams

100 kilogram = 220 pounds

1 ounce of gold = 28,349.5 milligrams

Density

• The measure of how much mass is contained in a given volume.

The formula of density is:

Density = Mass / Volume

Comparing Densities - Inferring: Which item has the greater density?

• The bowling ball• Since the bowling

bowl has a greater mass, it has a greater density, even though both balls have the same volume

Why is density expressed as a combination of two different units?

• Because density is actually made up of two other measurements – mass and volume – an objects density is expressed as a combination of two units.

Two Common Units For Density

• Grams per cubic centimeter (g/cm³)

• Grams per milliliter (g/mL)

Math Practice: What is the density of a wood block with a volume of 125 cm³ and a mass of 57 g?

Density = mass / volume

Density = 57 g / 125 cm³

Density = 0.46 g/ cm³

Math Practice: What is the density of a liquid with a mass of 45 g and a volume of 48 mL?

Density = mass / volume

Density = 45 g / 48 mL

Density = 0.94 g/mL

The density of a substance is the ______for all samples of that substance.

• Same

An object will float if it is _____ _____ than a surrounding liquid.

• Less dense

Applying Concepts: How could you use density to determine whether a bar of metal is pure gold?

• If the bar of gold has a density that is greater than or less than 19.3 g/cm³, then the sample is not pure gold.

Densities of Some Common

SubstancesSubstanc

eDensity(g/cm³)

Air 0.001Ice 0.9

Water 1.0Aluminum 2.7

Gold 19.3

Will an object with a density of 0.7 g/cm³ float or sink in water?

• An object that has a density of 0.7 g/cm³ will float in water (1 g/cm³) because it is less dense than water