Equations Revision Notes for OCR GCSE Chemistry A (Gateway Science Suite)

1.1.3 Equations

Compounds are represented using a formula made up of the symbols of the elements they contain.

Compound	Formula	No. of Elements	No. of Atoms
Sodium Chloride	NaCl	2	2 (1 Na, 1 Cl)
Sodium Hydroxide	NaOH	3	3 (1 Na, 1 O, 1 H)
Sodium Oxide	Na₂O	2	3 (2 Na, 1 O)
Sodium Sulphate	Na₂SO₄	3	7 (2 Na, 1 S, 4 O)
Calcium Carbonate	CaCO₃	3	5 (1 Ca, 1 C, 3 O)

infoNote

Worked Example Question: Question: What is the total number of atoms in the formula $(NH_4)_2SO_4$ , ammonium sulphate?

Solution:

$(NH_4)_2$ becomes…

2×NH_4 = (2×N)+(2×H4)=(2×N)+(8×H)

$SO_4$ becomes…

1×SO_4 = (1×S)+(1×O_4)=(1×S)+(4×O)

So…

\text{Total atoms} = (2 \times N) + (8 \times H) + (1 \times S) + (4 \times O) = 15 \text{ atoms}

Final Answer: Total = 15 Atoms