Question 11
What is the chemical formula of calcium hydroxide?
The right answer is the one marked above.
Ca(OH)2. Calcium has a valency of 2 and the hydroxide ion OH− a valency of 1, so two hydroxide ions are needed.
The brackets are not decoration. CaOH2 would say there is one oxygen and two hydrogens; Ca(OH)2 says there are two complete hydroxide groups. Whenever more than one polyatomic ion appears in a formula, it goes inside brackets.