Formulae for ionic compounds

4 marks · Normal

The following ions are present in a solution: Ca2+, OH–, Fe3+, Cl–.

(a)

Name the ionic compound formed when Ca2+ reacts with OH–.

[1 mark]

(b)

Write the formula of the compound formed between Ca2+ and OH–.

[1 mark]

(c)

Write the formula for the compound formed when Fe3+ reacts with Cl–.

[1 mark]

(d)

State why brackets are used in the formula for the compound formed between Ca2+ and OH–.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) calcium hydroxide

(b) Ca(OH)2

(d) Because there is more than one OH– ion; brackets show the whole ion is repeated.

Also accept: Accept 'calcium hydroxide' or 'iron(III) chloride' phrasing

Common mistakes: failing to use brackets for multiple OH–; incorrect balancing of charges

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · Correct name of compound: calcium hydroxide

A1 (b) · 1 mark · Correct formula for calcium hydroxide: Ca(OH)2

A2 (c) · 1 mark · Correct formula for iron(III) chloride: FeCl3

E1 (d) · 1 mark · Explains that brackets show more than one polyatomic ion.

Worked solution

a) Ca2+ and OH– form calcium hydroxide. b) Need two OH– ions to balance one Ca2+: Ca(OH)2. c) Fe3+ and Cl–: need three Cl– for one Fe3+: FeCl3. d) Brackets are used because there is more than one OH– ion; they show the group is repeated.