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
s
Electrical current
ampere
A
Temperature
Kelvin
K
Luminous 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 g
2.2 pounds (lb) = 1 kilogram (kg)
1 gram (g) = 1000 milligrams (mg) or 1 mg = 0.001 g
1 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) or
1 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 factors
There 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 g
1 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 value
Application of unit factor causes old units to cancel
Never forget to show units of any measurement –
unless it is a unit-less quantity