1. Relative atomic mass and relative molecular (formula) mass
Relative atomic mass (Ar) can be found from periodic table.
Relative molecular mass and relative formula mass (Mr) can be calculated from the addition of each components.
Notice: relative atomic mass and relative molecular (formula) mass have no unit, because they are relative values which compared to the mass of Carbon-12 atom.
Sample 1: Calculate Ar and Mr
Ar(Na) = 22.99
Ar(O2) = 16.00 × 2 = 32.00
Mr(NaCl) = 22.99 + 35.45 = 58.44
Mr(CuSO4·5H2O) = 63.55 + 32.07 + 4 × 16.00 + 5 × (2 × 1.01 + 16.00) = 249.72
Notice: “·” in formula represents “with” or “and”. CuSO4·5H2O represents that one CuSO4 formula unit contains five H2O molecules.
Difference between relative molecular mass and relative formula mass.
They have same symbol with Mr. Relative molecular mass is used to describe molecules, such as O2 and CO2, etc. Relative formula mass is usually to describe ionic compounds, such as NaCl and CuSO4, etc. Giant covalent structure, such as SiO2, has relative formula mass as well.
2. Molar mass
Molar mass (M) is useful in calculation, and it is used to describe the mass of one mole of substance. The value of molar mass for a specific substance is equal to the relative molecular mass, and the difference is molar mass has unit with g mol-1.
Sample 2: Calculate M
M(Na) = 22.99 g mol-1
M(O2) = 16.00 × 2 = 32.00 g mol-1
M(NaCl) = 22.99 + 35.45 = 58.44 g mol-1
M(CuSO4·5H2O) = 63.55 + 32.07 + 4 × 16.00 + 5 × (2 × 1.01 + 16.00) = 249.72 g mol-1
3. The amount of substance
The amount of substance (n, unit is mol) is used to describe the number of substance which is called Avogadro’s constant (L or NA). This number is equal to 6.02 × 1023. 1 mol = 6.02 × 1023
The amount of substance can be described any microscopic particles, including molecules, atoms, ions, even electrons and so on.
Sample 3: Find the number of particles for the following species
Number of water molecules in 0.100 mol of H2O.
6.02 × 1023 × 0.100 = 6.02 × 1022
Number of H atoms in 0.100 mol of H2O.
2 × 6.02 × 1023 × 0.100 = 1.20 × 1023
Number of ions in 0.100 mol of NaCl.
Definition
Relative atomic mass (Ar): average mass of one atom of one element compared to 1/12 the mass of one Carbon-12 atom.
Relative molecular (formula) mass (Mr): mass of one molecule (or formula unit) compared to 1/12 the mass of one Carbon-12 atom.
Molar mass (M): the mass of one mole of substance.
Empirical formula: the simplest whole number ratio of element in one compound.
Molecular formula: the actual number of atoms in one molecule.
Sample 2 Empirical
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