Units of Measurement And their Uses DR. C.’S PRE-AP CHEMISTRY FALL 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

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?  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 = ? 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

Volume Volume is the amount of 3D space an object occupies. The derived SI unit is m 3 The cubic centimeter, cm 3, 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 cm 3 1 L = 1000 cm 3

Density

Density is a characteristic physical property We can use density to help us identify a substance.

Conversion Factors

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