System International (SI)
In the late 1700’s the French invented the Metric System, based on:
1 meter = 1 ten millionth the distance from the North Pole to the Equator on a line passing
through Paris
1 liter = the volume of a cube measuring 1/10 m on each side
1 Kilogram = the mass of 1 liter of water
Celsius 0oC = mp of water; 100oC = bp of water
All units of the metric system increased or decreased by multiples of 10 from the base unit.
These changes were indicated by the use of prefixes. (See prefixes on PT)
In 1960 scientists at the “General Conference on Weights and Measures” adopted “Le System
International d’unites” (SI)
There are 7 basic SI units:
Meters (m) – length
Kilograms (Kg) – mass
Seconds (s) – time
Kelvin (K) temperature
Mole (mol) – amount
Ampere (A) – electrical current
Candela (Cd) – light intensity
All other units are derived from these 7 SI units.
Metric prefixes were retained for multiples of 10 larger or smaller than the base units.
One of the most important units in chemistry is the mole (mol).
1 mol = 6.02 x 1023 particles of any substance
i.e.
1 mol of oranges = 6.02 x 1023 oranges
1 mol of lead atoms = 6.02 x 1023 lead atoms
1 mol of sugar = 6.02 x 1023 molecules of sugar
6.02 x 1023 is called Avogadro’s Number
What makes this number so special and useful in Chemistry is that the Atomic mass of an
element given on the periodic table is actually the mass of 1 mol of that element in grams.
Molar Mass – the mass in grams of 1 mol of a substance
For: normal atoms the Molar Mass = the mass given on the periodic table
i.e.
1 mol of Carbon has a mass of 12.011 g
1 mol of sodium has a mass of 22.98977 g
diatomic elements the Molar Mass = 2 x the mass given on the PT
i.e.
1 mol of oxygen (O2) = 2(15.9994g) = 31.9988 g
1 mol of iodine (I2) = 2(126.9045) = 253.8090 g
Compounds the Molar Mass is the sum of atomic masses of all atoms in the compound.
i.e.
NaCl has 1 sodium for every chlorine
1 mol of sodium = 22.98977g
1 mol of chlorine = 35.4527 g
The molar mass of NaCl = 58.4425 g
Fe2(SO4)3 has:
2 mol Iron = 2(55.845) = 111.690 g
3 mol Sulfur = 3(32.066) = 96.198 g
12 mol Oxygen = 12(15.9994) = 191.9928 g
The molar mass of Fe2(SO4)3 = 399.881 g
Zn3(PO4)2 has:
3 mol Zinc = 3(65.409) = 196.227
2 mol Phosphorus = 2(30.97376) = 61.94752
8 mol Oxygen = 8(15.9994) = 127.9952
The molar mass of Zn3(PO4)2 = 386.170 g/mol
Compounds often absorb water which becomes part of their structure. These compounds are
called hydrates.
The amount of water is indicated after the compound’s formula with a dot between the
compound and the water. The molar mass of a hydrate must include the mass of the water
and the mass of the compound.
i.e.
CuSO4 • 5H2O
for mass of compound
1 mol of Copper = 63.546 g
1 mol of 32.066 g
4 mol Oxygen = 4(15.9994) = 63.9976 g
Total mass of compound = 159.6096 g
For mass of water
10 mol Hydrogen = 10(1.00794) = 10.0794 g
5 mol of Oxygen = 5(15.9994) = 79.997 g
Total mass of water = 90.0764 g
Total mass of Hydrate = 159.6096 g + 90.0764 g = 249.686 g/mol (3 DP)
CoK2(SO4)2• 6H2O
1mol Co = 58.9332
2 mol K = 2(39.0983) = 78.1966
2 mol S = 2(32.06)
8 mol O = 8(15.9994)
12 mol Hydrogen = 12(1.00794) = 12.09528
6 mol of Oxygen = 6(15.9994) = 95.9964
Total mass of Hydrate = 437.347 g/mol
Ni3(PO4) • 8H2O
3 mol Ni = 3(58.6934) = 176.0802
2 mol P = 2(30.97376) = 61.94752
8 mol O = 8(15.9994) = 127.9952
Total mass of Hydrate = 510.1452 g/mol
16 mol H = 16(1.00794) = 16.12704
8 mol O = 8(15.9994) = 127.9952