CHEMISTRY IS LARGELY A QUANTITATIVE SCIENCE

The recording of a quantitative observation (some kind of numerical value) in experiments involves ACCURACY and PRECISION. These are not the same thing. It is necessary to record data with both the appropriate number of SIGNIFICANT DIGITS and the CORRECT UNITS.

Systems of measurements

A. English System - what the United States uses today derived from the 'Kings" feet (Old England)

Units 1 foot (“big feet")

1 inch (knuckle)

1 yard (King Henry I - nose to thumb)

System Internationale (S.I.)

The creation of the decimal Metric System at the time of the French Revolution and the creation of two platinum standards representing the meter and the kilogram was the first step in the development of the present International System of Units.

Prefixes

The seven fundamental units of measurement

Physical property Name of unit Symbol

Length meter m

Mass kilogram kg

Time second sElectrical current ampere A

Temperature Kelvin KLuminous intensity candela Cd

Amount of substance mole mol

Length

Length is a fundamental unit. In the metric system, the meter is a the standard unit for measuring length. It is a little longer than a yard. The standard unit of metric length is kept in Paris. It is equal to 39.37 inches. Since the meter is equal to 100 centimeters (cm), then 1 inch is equal to 2.54 cm. Comparisons:

centimeter - width of your fingernail kilometer - 0.60 miles millimeter - thickness of a dime

Mass

Mass is a fundamental unit; it is a measure of the quantity of matter present. Although we typically use the words mass and weight interchangeably in normal conversation, mass and weight actually have different meanings. Weight includes the effect of gravity on mass. In a space capsule, your weight would be zero (absence of gravity) but your mass would be the same value as it was on earth. The standard unit of mass is the kilogram (kg). Conversions factors:

1 pound = 454 g2.2 pounds (lb) = 1 kilogram (kg)1 gram (g) = 1000 milligrams (mg) or 1 mg = 0.001 g1 kilogram (kg) = 1000 grams (g) or 1 g = 0.001 kg

Volume

– volume is a derived unit (using length units). It is a measure of space. In simple terms, it is a measure of three (3) length units in three dimensions (breadth, length and width). The standard unit of measurement is the meter cubed (m3) or the centimeter cubed (cm3).In chemistry, volume is typically measured for a liquid. The standard unit of measurement for a liquid is the liter (L). The liter is similar to a quart in the English system. Conversion factors:

1 liter (L) = 1.06 quarts (qt)1 quart (qt) = 0.946 liters (L)1 liter (L) = 1000 milliliters (ml) or1 milliliter (ml) = 0.001 liter (L)1 milliliter (ml) = 20 drops At room temperature, 1 milliliter (ml) of water = 1 cm3. (or 1 CC)

Temperature

A fundamental unit. There are three(3) temperature scales BPH20 FPH2O Difference (1) Fahrenheit (F) 212° 32° 180°(2) Celsius (centigrade) (C) 100° 0° 100°(3) Kelvin (K) 373 273 100

Conversion:Kelvin to Celsius (K -> C) C = K - 273 Celsius to Kelvin (C -> K) K = C + 273 Celsius to Fahrenheit (C -> F)

i multiply C x 9/5 ii Add C x 9/5 + 32

Fahrenheit to Celsius (F -> C) i add F - 32 ii multiply (F - 32) x 5/9

Derived units

Derived units can be resolved into combinations of fundamental units

Area and volume involve only the units of length

Most involve combinations of different units

Dimensions

It is important to keep track of the dimensions in derived units.

All the more when making conversions:1 cm = 10 mm

1 cm2 = 100 mm2

1 cm3 = 1,000 mm3

Density

Mass divided by volume

Units are:g/cm3(solid)

g/mL (liquid)

g/m3 (gas)

Density and temperature

In most cases, the density of a substance decreases with temperature. (Why is that?)

Water provides a critical exception to the rule

Conversion factors

Making conversions between different units is very important

Always keep track of the units

Make use of unit factorsThere are 1000 mg in 1 g (conversion factor)

1000 mg/1 g = 1; 1 g/1000 mg = 1 (unit factors)

There are two unit factors for any conversion

Unit factors at work

How many grams are there in 2680 mg of sucrose?

We know there are 1 000 mg in 1 g1 g/1000 mg = 1 (unit factor)

? g = amount in mg x unit factor? g = 2680 mg x 1 g/1000 mg = 2.680 g

Unit factor has value of 1 – no change in valueApplication of unit factor causes old units to cancelNever forget to show units of any measurement – unless it is a unit-less quantity