Formulae for ionic compounds
2 marks · EasyAmmonium sulfate has the formula (NH4)2SO4. Explain why two ammonium ions are needed in this compound.
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Because each ammonium ion (NH4+) has a 1+ charge and each sulfate ion (SO4^2−) has a 2− charge, so you need two NH4+ to balance the sulfate's 2− charge.
Also accept: Because two positive charges from two NH4+ are needed to balance the two negative charges from one SO4^2− ion.; To make the overall charge zero, as the compound must be neutral.
Common mistakes: Thinking all formulae must have two ammonium ions; Not stating that charges must cancel
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Marking points
M1 · 1 mark · Mentions correct charges: NH4+ is 1+ and SO4^2− is 2−.
C1 · 1 mark · Clearly explains that two NH4+ are needed to balance the 2− charge, so the compound is neutral.
Worked solution
Each ammonium ion (NH4+) has a 1+ charge. Sulfate (SO4^2−) has a 2− charge. To cancel out the 2− charge, two ammonium ions (total 2+) are needed, so (NH4)2SO4 is neutral overall.