Chemical Formulae, Equations and Calculations
Principles of Chemistry · 6 question types
- The (Mr) of a substance is the sum of the relative atomic masses of every atom in its chemical formula
- The relative atomic mass (Ar) of each element is read from the periodic table (it's the larger of the two numbers in each box)
- For an element written as just its symbol (e.g. Fe), Mr = Ar
- For a compound, multiply each Ar by the number of atoms of that element in the formula, then add the products
| Substance | Atoms present | Calculation | Mr |
|---|---|---|---|
| H₂O | 2 H + 1 O | (2 × 1) + 16 | 18 |
| CO₂ | 1 C + 2 O | 12 + (2 × 16) | 44 |
| CaCO₃ | 1 Ca + 1 C + 3 O | 40 + 12 + (3 × 16) | 100 |
| H₂SO₄ | 2 H + 1 S + 4 O | (2 × 1) + 32 + (4 × 16) | 98 |
| Mg(NO₃)₂ | 1 Mg + 2 N + 6 O | 24 + (2 × 14) + (6 × 16) | 148 |
- By the law of conservation of mass, the total Mr on the reactant side (with each Mr multiplied by its coefficient) equals the total Mr on the product side
Counting atoms and calculating a relative formula mass
Question: Calculate the relative formula mass (Mr) of a compound from its formula, or give the number of atoms, or of different elements, in the formula (1–2 marks).
An Mr calculation is on 11 of the 23 papers and an atom or element count on 9. For Mr, multiply each relative atomic mass by the number of that atom and add them: a one-mark version wants only the total; a two-mark version gives one mark for the written sum and one for the total; one scheme followed a wrong sum through to the total mark and another did not, so get the sum right. Units are ignored. Brackets are the trap: in Mg(NO₃)₂ everything inside the bracket is doubled, and a sum that leaves the bracket undoubled loses the first mark and keeps the second only where the total is followed through from your own sum.
For the counts, add every subscript for the atoms (a dot formula's water counts too: CuSO₄·5H₂O has 21 atoms) and count each element symbol once for the elements (four in that formula). "Different elements" and "atoms" are different numbers.
Mr of a compound containing brackets
A student needs the relative formula mass of calcium hydroxide, Ca(OH)₂. Calculate it.
Solution:
- Identify all atoms: 1 Ca, and the bracket (OH)₂ means 2 O and 2 H
- Ar values: Ca = 40, O = 16, H = 1
- Mr = (1 × 40) + (2 × 16) + (2 × 1)
- Mr = 40 + 32 + 2 = 74