An inorganic salt was analysed to determine its composition. The compound gave a lilac flame in a flame test. After dissolving in water and adding sodium hydroxide solution, a green precipitate was formed. After dissolving in water, acidifying with dilute hydrochloric acid and adding barium chloride solution, a white precipitate was formed. Which of the following could be the formula of the salt?

Answer:

10) H

Potassium gives a lilac flame in a flame test.
Iron (II) gives green precipitate when added to sodium hydroxide solution.
A white precipitate in the barium chloride test indicates the presence of sulphate ions.
Options F and H could be correct as they both have potassium, iron (II) and sulphate ions. However, the correct answer is H as there would need to be two potassium atoms to have an overall charge of 0.
There are two sulphate ions and each sulphate ion has a charge of 2-. So, two sulphate ions would have charge of 4-
Iron (II) has charge of 2+
Potassium has charge of 1+
Hence, there would need to be two potassium ions to balance the charges.
It is important for you to learn the flame test results and the precipitation reaction results as part of your theory.