The correct answer is C because:

It has a percentage by mass of 44.8% which is more than the other fertiliser compounds
To answer this question, you need to determine the percentage by mass of the essential elements for all of the fertilisers
Remember: The essential elements are nitrogen, phosphorous and potassium (NPK)
To calculate the percentage by mass, use the following equation:
- % by mass = $\frac{total mass of the element in the compound}{M_{r} of the compound} \times 100$
For (NH₄)₂SO₄
- % by mass = $\frac{2 \times 14}{2 \times (14 + (4 \times 1)) + 32 + (4 \times 16)} \times 100$ = 21.2%
For NH₄NO₃
- % by mass = $\frac{2 \times 14}{(2 \times 14) + (4 \times 1) + (3 \times 16)} \times 100$ = 35.0%
For K₂SO₄
- % by mass = $\frac{2 \times 39}{(2 \times 39) + 32 + (4 \times 16)} \times 100$ = 44.8%
Ca₃(PO₄)₂
- % by mass = $\frac{2 \times 31}{(3 \times 40) + 2 \times (31 + (4 \times 16))} \times 100$ = 20%

Fertilisers (Cambridge O Level Chemistry)

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