measurements in chemistry chemistry notes #2 unit #1

Measurements in Chemistry

Chemistry Notes #2Unit #1

How do you describe measurements in chemistry?

• Quantitative data….

Some Examples of Quantitative data

• Mass• Length• Volume• Temperature• Time• Density• Moles• Energy

Types of Units• All measurements are measured using metric

system…• Units fall into two categories: – Base- units produced by solely one measurement• Mass• Temperature• time

– Derived- units produced by the combination of more than measurement • Volume- (length X width X height)• Density – (mass/volume)

A few reminders for taking measurements…

• Measurements are estimated…

• That estimation is called…– Significant figures

• Units are required… Without them its like not including a last name.

• Conversion might be needed…

2.4 Measurement and Significant 2.4 Measurement and Significant FiguresFigures

• Every experimental measurement has a degree of uncertainty.

Chapter Two 6

Learning CheckLearning Check

What is the length of the wooden stick?1) 4.5 cm 2) 4.54 cm 3) 4.547 cm

Measured NumbersMeasured Numbers

• Measured numbers contain error…– This is called estimation of digits

• The last significant figure is only the best possible estimate.

8

Chapter Two 9

Below are two measurements of the mass of the same object. The same quantity is being described at two different levels of precision or certainty.

Sig Figs and Making calculations

• Remember rules for sig figs…• Two types of numbers:• Whole numbers

1) Start on right side of #2) Count every digit from first non-zero on

• Numbers with Decimals1) Start on left side of #2) Count every digit from first non-zero on

Calculations

• Addition and Subtraction:– Answer can have no more places past the decimal

then the measurements you start with in the calculation.

• Multiplication and Division:

__ ___ __

Addition and SubtractionAddition and Subtraction

.56 + .153 = .713

82000 + 5.32 = 82005.32

10.0 - 9.8742 = .12580

10 – 9.8742 = .12580

.71

82000

.1

0

Look for the last important digit

Calculations continued:

• Multiplication and Division:– Answer can have no more significance then the

least amount from the starting measurements.

Multiplication and divisionMultiplication and division

32.27 1.54 = 49.6958

3.68 .07925 = 46.4353312

1.750 .0342000 = 0.05985

3.2650106 4.858 = 1.586137 107

6.0221023 1.66110-24 = 1.000000

49.7

46.4

.05985

1.586 107

1.000

Scientific Notation

• Examples of Scientific Notation to standard notation:

Scientific NotationExamples of standard notation to scientific

notation.

Solving Calculations in Chemistry

• Two types of calculations:– Plugging numbers into a calculation– Converting quantities from one unit into another

Examples of Calculations:

P1V1 = P2V2 V1/T1 = V2/T2 D= M/V

S= D/T

Density Practice: • What is the density of a piece of wood that has a

mass of 25.0 grams and a volume of 29.4 cm3?

• A cup of gold colored metal beads was measured to have a mass 425 grams. By water displacement, the volume of the beads was calculated to be 48.0 cm3. Given the following densities, identify the metal.

Gold: 19.3 g/mLCopper: 8.86 g/mL

Bronze: 9.87 g/mL

Honors density problem:

• A little aluminum boat (mass of 14.50 g) has a volume of 450.00 cm3. The boat is place in a small pool of water and carefully filled with pennies. If each penny has a mass of 2.50 g, how many pennies can be added to the boat before it sinks?

Basic Conversions

• Calculations from one unit into another

• Ladder of Conversions allows us to go from one unit to another.– Middle school and physical science way of going

from one unit to another! Just move up and down the ladder

Conversion ladder

The Chemistry way- Dimensional Analysis

• A step by step process that allows you to complete basic conversions- Like a system of checks and balances.

• Requires use of a conversion factor:– A conversion factor is a fraction that allows you to

go from one unit to another. – Conversion factor allows all units to cancel so you

are left with the units you are trying to reach.

Practicing conversion Factors

• Write the conversion factors for the following relationships:

a) 1000mL= 1 L

a) 2 wheels= 1 bicycle

Using Dimensional Analysis Example:

4.5 L = ____ mL Steps: 1)Identify your known and unknown.2)Determine relationship between known and unknown.3)Write out conversion factors for calculation.4)Draw chart, writing out your known and inserting your conversion factor.

Known= 4.5 LUnknown= ___ mL1 L = 1000 mL1000mL or __1L__ 1 L 1000mL

4.5 L x 1000m L = .0045 mL 1 L

A few practice:

25 cg = ___ g

15 wheels = _____ bicycle

How many seconds are in 1 day?

So why does the method matter?

• What if you are converting from a unit that is on the ladder into a unit that is not?

• For example in chemistry: – Works with grams, moles, molecules

– If I measured out 14.1 g of Na, how many moles of Na do I have?

So how would you solve that question?

• How do you know number of moles? • That leads us to another quantitative piece of

data… Called molar mass.

Molar Mass6.02 X 1023 particles = 1 mole = grams grams of a substance = atomic mass of substance. • For example: – The molar mass of Sodium:• 22.99 g of Na = 1 mole of Na

– The molar mass of Chlorine:• 35.45 g of Cl = 1 mole of Cl

– The molar mass of sodium chloride:• (22.99g of Na + 35.45 g of Cl) = 1 mole of NaCl

Or 58.44 g of NaCl = 1 mole of NaCl

Determine the molar mass of the below elements or compounds:

• F

• O2

• H20

• Fe

Solving using molar mass:

• How many moles of Na are in a sample of 14.1g of Na.

• Use the conversion rules to solve this problem: