Units of MeasurementAnd their UsesDR. C.’S PRE-AP CHEMISTRYFALL 2015

Lesson Objectives:• Distinguish between a quantity, a unit, and a measurement standard

• Name the SI units for length, mass, time, volume and density

• Transform a statement of equality into a conversion factor

• Distinguish between precision and accuracy

• Determine the number of significant figures in a number

• Perform mathematical computation using significant figures

Accuracy vs. Precision• Accuracy refers to the closeness of a measurement to the accepted value.

• Precision refers to the closeness of a set of measurements to each other.

Percent Error• Percent error is a mathematical way to determine how

accurate a measurement is.

Percentage error = x 100

Significant Figures

Significant Figures Rules

• All non-zero numbers are significant.

• Leading zeros are not significant

• Zeros between two non-zero numbers are signficant

• Trailing zeros are where things get confusing…

Trailing zeros are not significant unless:

1. They are after a decimal

2. There is a decimal at the end of the number

How many significant figures?

• 28.6

• 3440

• 3440.

• 0.0004604

• 500000000000000000 this one makes my son upset! =)

Significant Figures

“Doing Math” with sig figs

Always do the calculation and then round to the correct number of sig figs.

• Addition or Subtraction: fewest number of decimal places

18.4 + 3.45 = ?

• Multiplication or Division: fewest total # of sig figs

2.4 x 15.82

Quantity• Measurements represent quantities.

• A quantity is something that has magnitude, size or amount.

• Measurement ≠ quantity

• The teaspoon is a unit of measurement

• Volume is a quantity

• The choice of unit depends on the quantity being measured

7 Fundamental Base Units

Mass vs. Weight

Reminder!! (You should already have this in your notes)

•Mass is the measure of the amount of matter present.

•Weight is a measure of the gravitational pull on matter.

• The SI base unit for mass is the kilogram

Length

•Length is a measure of distance.

•The SI unit for length is the meter.

•The kilometer is used for longer distances, while centimeter is often used to express shorter distances.

Derived SI Units

•Combinations of SI base units form derived units.• A Joule (Unit for energy) is

Volume• Volume is the amount of 3D space an object occupies.

• The derived SI unit is m3

• The cubic centimeter, cm3, is often used

• Most often in chemistry you will see the liter (L) or milliliter (mL) even though it is a non-SI unit.

• 1 mL = 1 cm3

• 1 L = 1000 cm3

Density

•Density is the ratio of mass to volume.

density =

• The SI unit for density is kilogram per cubic meter, kg/m3

• In chemistry most often we will use grams per milliliter, g/mL

Density is a characteristic physical property

• We can use density to help us identify a substance.

Conversion Factors

•A conversion factor is a ratio derived from the equality between two different units that can be used to convert from one unit to another.

Example: There are four quarters in a dollar.

is the same mathematically as

Dimensional Analysis• Dimensional Analysis is a mathematical technique that allows you

to use units to solve problems involving measurement.

Steps to solving dimensional analysis problems:

1. Write the problem in question mark form.

2. Set up a “Picket Fence”

3. Start with the known (given) quantity

4. Cancel the unwanted unit by sliding it diagonally to the next part of the fence

5. Use a conversion factor to convert from one unit to another

6. Continue to cancel unwanted units until you end with the desired unit.

7. Multiply and divide

Example (Done on white board)

How many seconds are there in one year?

Another Example

Would you be breaking the speed limit in a 40 mi/h zone if you were traveling at 1 km/min?

1 km = 0.62 miles

Lesson Objectives:• Distinguish between a quantity, a unit, and a measurement standard

• Name the SI units for length, mass, time, volume and density

• Transform a statement of equality into a conversion factor

• Distinguish between precision and accuracy

• Determine the number of significant figures in a number

• Perform mathematical computation using significant figures