Chapter 3

One of the key parts of the Scientific Method is the ability to make measurements. If I told you a measurement was 59.7. What would be your response? The metric system is the one used in science. The units are called SI units. SI base units are listed on page 74.

Measurements in Chemistry Quantity Unit Symbol length meter m mass kilogram kg time second s current ampere A temperature Kelvin K amt. substance mole mol

Some metric prefixes are listed on page 75. You must know the following prefixes: Kilo = 1000 Deci = 0.1 Centi = 0.01 Milli = 0.001

Units of Measurement Common Conversion Factors Length Volume 1 m = 39.37 inches 2.54 cm = 1 inch Volume 1 liter = 1.06 qt 1 qt = 0.946 liter

The SI base units are used to derive other units The SI base units are used to derive other units. One of the common derived units is used to measure volume. The SI unit for volume is the cubic meter. This is not a very practical unit to use in the lab because it is so large.

One important physical property of matter is density. Density = mass/volume Every substance has its own unique density. See page 81 for a list.

There is some interesting info in the table. Notice the density of ice is 0.92g/cm3 and for water it’s 0.998g/ml. Does this difference have any significance? Since the density formula has 3 variables, three types of problems are possible. D = m/v

1. Given mass and volume, find density A substance has a mass of 28.4 grams and a volume of 20.5cm3. Find its density. 2. given density and volume, find mass (g). D = m/v so m= D x v The density of silver is 10.5g/cm3. Find the mass of a block of silver with a volume of 45.0cm3.

3. Given the density and mass, find the volume of a substance. D = m/v so v= m/D Find the volume of a piece of iron that has a mass of 147 g.

Dimensional Analysis It is important to be able to convert one unit into another. We will make use of conversion factors (also known as unit factors). For example how many grams are there in 2.5Kg? What you need to know is how many grams are in 1 Kg. We know that there are 1000 g in 1 Kg. 2.5 Kg x 1000/1Kg = 2500g.

Some for you to try: a. 2.65 g to Kg b. 18.7 cm to m c. 2652 mg to Kg

Accuracy and precision Accuracy refers to the proximity of a measurement to the true or accepted valueof a quantity. Precision refers to the proximity of several measurements of the same thing to each other. See p64 for examples.

If we happen to know the true or accepted value for a measurement, then we can calculate the per cent error of our measurement. Percent error = (measured value – accepted value) X 100 accepted value

Every measurement has some uncertainty associated with it. See page 66 Every measurement has some uncertainty associated with it. See page 66. In every measurement there is a known or certain quantity and an estimated quantity. In every measurement all the numbers are significant. Many measuring instruments allow us to make an estimate of the last number in a measurement.

Use of Numbers Exact numbers Accuracy Precision 1 dozen = 12 things how closely measured values agree with the correct value Precision how closely individual measurements agree with each other

Use of Numbers Significant figures digits believed to be correct by the person making the measurement Measure a mile with a 6 inch ruler vs. surveying equipment Exact numbers have an infinite number of significant figures 12.000000000000000 = 1 dozen because it is an exact number

Significant Figures - Rules Use of Numbers Significant Figures - Rules Leading zeroes are never significant 0.000357 has three significant figures Trailing zeroes may be significant must specify significance by how the number is written 1300 nails - counted or weighed? Use scientific notation to remove doubt 2.40 x 103 has ? significant figures

How many significant figures are present in each of the following measurements. 20.4 cm 970 bricks 92.00Kg 946025.3709 miles

Use of Numbers Multiplication & Division rule Easier of the two rules Product has the smallest number of significant figures of multipliers

Use of Numbers Multiplication & Division rule Easier of the two rules Product has the smallest number of significant figures of multipliers

Use of Numbers Multiplication & Division rule Easier of the two rules Product has the smallest number of significant figures of multipliers

Use of Numbers Addition & Subtraction rule More subtle than the multiplication rule Answer contains smallest decimal place of the addends.

Use of Numbers Addition & Subtraction rule More subtle than the multiplication rule Answer contains smallest decimal place of the addends.

Use of Numbers Addition & Subtraction rule More subtle than the multiplication rule Answer contains smallest decimal place of the addends.