Chapter 2 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 metric system is the one used in science. The units are called SI units-we will see that not all the units we will use are SI units. SI base units are listed on p 34.

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

Measurements in Chemistry Metric Prefixes Name Symbol Multiplier mega M 106 kilo k 103 deka da 10 deci d 10-1 centi c 10-2

Measurements in Chemistry Metric Prefixes Name Symbol Multiplier milli m 10-3 micro  10-6 nano n 10-9 pico p 10-12 femto f 10-15

You must know the following SI prefixes: Kilo-1000 Deci-0.1 Centi 0.01 Milli 0.001 Others will be provided.

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. Some are listed on page 36. One of the common derived units is volume. The SI unit for volume is the cubic meter (V=lxwxh) m3. This is not a very practical unit to use in the lab.

Volume The most commonly used metric units for volume are the liter (L) and the milliliter (mL). A liter is a cube 1 dm long on each side. A milliliter is a cube 1 cm long on each side.

One important physical property of matter is density . Density = mass/volume Every substance has its own unique density. See p 38 for a list.

There is some interesting info in the table. Notice the density of ice: 0.92g/cm3 and for water 0.998g/mL What do you think this means?

Since the density formula has 3 variables, 3 types of problems are possible. D = m/V

1. given mass and volume-find density a substance has a mass of 23.2 grams and a volume of 18.5 cm3. Find its density. 2. given density and volume, find mass (g) D = m/V so m=D x V The density of silver is 10.5 g/cm3. Find the mass of a block of silver with a volume of 40.0cm3.

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 147grams. ( density of iron = 7.86 g/cm3)

Dimensional Analysis It is important to be able to convert one unit to another. We will make use of conversion factors (also known as unit factors). For example: how many grams are there in 25 kg? What you need to know is how many grams there are in 1 kg. We know (or will know) that there are 1000g in one Kg (or 1 kg contains 1000 grams.

25 kg. X 1000 g = 25000 g. 1 kg

Some for you to try: a. 1.34 g to kg b. 15.2 cm to m c. 2580. mg to kg

Accuracy versus Precision Accuracy refers to the proximity of a measurement to the true value of a quantity. Precision refers to the proximity of several measurements to each other.

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

Every measurement has some uncertainty associated with it. See page 46 Every measurement has some uncertainty associated with it. See page 46. In every measurement there is a known or certain quantity and an estimated quantity. In every measurement all the numbers are significant.

Measurements & Calculations Many measuring instruments allow us to make an estimate of the last number in a measurement.

Use of Numbers Piece of Black Paper – with rulers beside the edges

Use of Numbers Piece of Paper Side B – enlarged How long is the paper to the best of your ability to measure it?

Use of Numbers Piece of Paper Side A – enlarged How wide is the paper to the best of your ability to measure it?

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

Use of Numbers If you measured it-it’s significant Significant figures digits believed to be correct by the person making the measurement If you measured it-it’s significant 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

Try these: How many significant figures are present in each of the following measurements 236.5 g ______ 20.4 cm _____ 970 bricks _____ 92.00 kg ____ 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.