GCSE / O Levels Chemistry

In order to decide which would be the better nitrogenous fertiliser, a student calculates the
percentage by mass of nitrogen in both ammonium sulfate and ammonium nitrate.
Which row gives the correct results?
percentage by
mass of nitrogen in
ammonium sulfate
percentage by
mass of nitrogen in
ammonium nitrate
A
10.6
17.5
B
10.6
35.0
21.2
35.0
D
21.2
17.5


Choice C is correct because:

First, calculate the molar mass of both compounds:

Ammonium sulfate, (NH4)2SO4 : 2 x [(1.01 g/mol x 2) + 14.01 g/mol] + 32.06 g/mol + (4 x 16.00 g/mol) = 132.14 g/mol

Ammonium nitrate, NH4NO3 : (1.01 g/mol x 2) + 14.01 g/mol + 14.01 g/mol + (16.00 g/mol x 3) = 80.05 g/mol

Next, calculate the mass of nitrogen in both compounds:

Ammonium sulfate, (NH4)2SO4 : 2 x 14.01 g/mol = 28.02 g/mol

Ammonium nitrate, NH4NO3 : 2  x14.01 g/mol = 28.02 g/mol

Finally, find the percentage by mass of nitrogen in both compounds:

Ammonium sulfate, (NH4)2SO4 : (28.02 g/mol / 132.14 g/mol) x 100 = 21.2%

Ammonium nitrate, NH4NO3 : (28.02 g/mol / 80.05 g/mol) x 100 = 35.0%

Choices A, B, and D are incorrect because they do not correctly identify the percentage by mass of nitrogen in both or one of the compounds.

Relative Masses of Atoms and Molecules | 2021 - O/N - 5070/11 | Q25