Moles Question Solved

Question

A compound of formula M3N contains 0.673 g of nitrogen per gram of the metal M. What is the relative atomic mass of M? Identify M.

Solution

In M3N, 1 mole of nitrogen would be combined with 3 moles of M.

This means, in terms of mass, that 14 g of nitrogen would be combined with a mass of M, equal to that of 3 moles of M.

If 1 g of M was combined with 0.673 g of nitrogen.

Then 14 g of nitrogen would be combined with 20.802 g of M.

From the first line, above, this mass of M is that of 3 moles of M.

Hence molar mass of M = 6.934 g mol-1 and its relative atomic mass is 6.934.

Identity of M is lithium.

Chlorine Bleach

Hypochlorous acid, HClO, forms when chlorine dissolves in water. In this reaction, half of the chlorine is oxidized to hypochlorite ion and half is reduced to chloride ion in a disproportionation reaction.

Cl2(g) + 2H2O(l) ⇌ H3O+(aq) + HClO(aq) + Cl−(aq)

If Cl2 is dissolved in cold aqueous NaOH instead of in pure water, hypochlorite ion and chloride ion form.

Cl2(g) + 2OH−(aq) ⇌ ClO−(aq) + Cl−(aq) + H2O(l)

Under basic conditions, the equilibrium lies far to the right. The resulting alkaline solution is the “liquid bleach” used in home laundries. The bleaching action of this solution is a result of the oxidizing ability of ClO−. Most dyes are colored organic compounds, and hypochlorite ion oxidizes dyes to colorless products.

When calcium hydroxide is combined with Cl2, solid Ca(ClO)2 is the product. This compound is easily handled and is the “chlorine” that is sold for swimming pool disinfection.

This information is relevant to the January 2016 P2 and can be used to answer Q 2(a).