Formulae, Equations and Amount of Substance
The Mole and Relative Masses
Pearson Edexcel International A Level Chemistry
Amount of substance and molar mass
amount (mol) = mass (g) / molar mass (g mol−1)
n = m / M m = n × M
| Quantity | Symbol | Unit |
|---|---|---|
| amount of substance | n | mol |
| mass | m | g |
| molar mass | M | g mol−1 |
| Avogadro constant | L | mol−1 |
- Molar mass is the Mr in grams per mole: write its unit as g mol−1.
- A mass given in mg is divided by 1000 before it goes into n = m / M.
- Write the mass in g and divide by M. Never write a mass in mg or g treated as an amount in moles.
The Avogadro constant
number of particles = amount (mol) × Avogadro constant
N = n × L L = 6.02 × 1023 mol−1
- The particle counted is the one named: molecules, atoms or ions.
- Atoms in a diatomic molecule: amount of atoms = amount of molecules × 2, then multiply by L.
Relative formula mass
- Add the Ar of every atom in the formula: a name becomes a formula first.
- For an ionic compound or a hydrate the total is the relative formula mass, used in place of Mr.
- Water of crystallisation is added as whole H2O units: x × 18.0 for xH2O.
- Mr and the relative formula mass have no unit; the molar mass carries g mol−1.
Identifying an element
Method: naming the unknown element in a formula
- Find the amount in moles of the compound from the data given.
- Divide the mass of the compound by that amount, M = m / n, for the molar mass in g mol−1.
- Subtract the Ar values of the known part of the formula, leaving the Ar of the unknown element.
- Name the element from that Ar, and check it forms an ion of the charge the formula needs.
Parts per million
ppm stands for parts per million.
ppm = (amount of the part / amount of the whole) × 106
percentage = ppm / 104
- By mass, for a solute in a solution: mass of solute / mass of solution = ppm / 106.
- By volume or amount, for a gas in a mixture such as the air: amount of the gas = ppm / 106 × total amount.
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