Exam Questions Stoichiometry

Practise How to Write Chemical Formula

Can you infer the formulae of ionic compounds, based on the charges of their constituent ions?

Basic Questions

Write the formula of zinc chloride.

ZnCl2

Write the chemical formula of iron(III) oxide.

Fe2O3

Marker’s note: the Roman numeral III refers to the charge of iron, which is a transition metal with variable charge.

Write the chemical formula of calcium hydroxide.

Ca(OH)2

Marker’s note: hydroxide is a polyatomic anion, OH

Write the chemical formula of ammonium chloride.

NH4Cl

Marker’s note: ammonium is a polyatomic cation, with the formula NH4+

Challenging Questions from Prelim Papers

An unknown element X forms an ionic compound with oxygen. Given that the formula of the compound is X2O3, what is the charge of the ion of X?

Answer: 3+

We know that three oxide ions, O2- will have a combined charge of 6-. This is cancelled out by simply 2 cations, suggesting that X3+ has a charge of 3+.

The chemical formula of barium oxide is BaO. The chemical formula of sodium manganate(VII) is NaMnO4. Predict the chemical formula of barium manganate(VII).

Answer: Ba(MnO4)2

Don’t know the charges of barium ion and manganate(VII) ion? Panic not, as this is an inference question and you are not supposed to know anyway. Start from the charges of common ions that you do know: oxide, O2- and sodium, Na+.

Using the chemical formula of BaO, the charge of O2- requires only 1 barium ion to cancel out, so it must have a charge of 2+. Likewise, using the chemical formula of NaMnO4, the charge of Na+ requires only 1 manganate(VII) ion to cancel out, so it must have a charge of 1-.

Thus, the 2+ charge of Ba2+ cation will need 2 MnO4 anion to cancel out. This gives the chemical formula of Ba(MnO4)2.

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