The table compares three nitrogen fertilisers. Use these relative atomic masses: H = 1, C = 12, N = 14, O = 16, S = 32. Fertiliser Formula Mr % nitrogen by ...

This question tests relative formula mass and percentage composition, then uses the results to compare fertilisers. The completed table is shown first, followed by the working.

(a) For (NH₄)₂SO₄, add the masses inside the brackets first, then double, then add S and 4 O:

\[ M_r = 2\times(14 + 4\times1) + 32 + 4\times16 = 2\times18 + 32 + 64 = 36 + 32 + 64 = 132 \]

1 mark for the working, 1 mark for 132.

(b) There are 2 nitrogen atoms, mass \( 2\times14 = 28 \) [1]. Then:

\[ \%\,N = \frac{28}{132}\times100 = 21.2\% \]

1 mark for 21.2%.

(c) Urea CO(NH₂)₂ also has 2 nitrogen atoms, mass \( 2\times14 = 28 \), and M_r = 60:

\[ \%\,N = \frac{28}{60}\times100 = 46.7\% \]

1 mark for the 28/60 setup, 1 mark for 46.7%.

(d) Comparing the percentages (35.0, 21.2, 46.7), the best source of nitrogen per kilogram is urea [1].

(e) Urea is cheaper to transport for the same mass of nitrogen because it has a higher percentage of nitrogen [1], so a smaller mass of urea carries the same mass of nitrogen, which lowers the transport cost [1].

(f) The element in ammonium sulfate but not in ammonium nitrate is sulfur [1].

(g) A soluble fertiliser is more useful because it dissolves in soil water so its ions can be absorbed by the roots [1]; an insoluble one stays out of reach of the plant.

(h) The equation for making ammonium nitrate is:

\[ NH_3 + HNO_3 \rightarrow NH_4NO_3 \]

1 mark for correct formulae, 1 mark for balancing (it is already balanced as written).

Exam tip: percentage of an element = (total mass of that element divided by M_r) times 100; the higher the % nitrogen, the more nitrogen you carry per kilogram.