Salts and Their Uses: Quick Revision

A brief note on Salts and Their Uses. Read it before a test. Try each question at the end before you open its answer.

Salts and their pH

Substances from common salt

• Chlor-alkali: $2\text{NaCl} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{Cl}_2 + \text{H}_2$. Chlorine at the anode, hydrogen at the cathode.
• Bleaching powder: $\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$.
• Baking soda: $\text{NaHCO}_3$, a mild base; an antacid, and in baking powder with a mild acid.
• Washing soda: $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$; softens hard water.

Water of crystallisation

Blue $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ turns white on heating and blue again with water. Gypsum, $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$, heated to $373\ \text{K}$ gives plaster of Paris, $\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O}$, which sets hard with water.

A worked example

What mass of water is there in $50\ \text{g}$ of Epsom salt, $\text{MgSO}_4 \cdot 7\text{H}_2\text{O}$ ($246\ \text{g/mol}$)?

1. Water in one unit. $7 \times 18 = 126\ \text{g/mol}$.
2. Share. $\dfrac{126}{246}$ of the mass.
3. Mass. $50\ \text{g} \times \dfrac{126}{246} = 25.6\ \text{g}$, about half.

Practice with new numbers

A program to try

The program works out what one tonne of salt gives in the chlor-alkali process.

Mistakes to avoid

• Thinking every salt is neutral. It depends on the acid and the base.
• Mixing up baking soda and washing soda. $\text{NaHCO}_3$ against $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$.
• Overheating gypsum. It then loses all its water and will not set.
• Leaving plaster of Paris open to damp air.

Check yourself

1. What is the chemical name of washing soda?

2. A solution of ammonium nitrate turns blue litmus red. Why?

3. Which gas is given off at the cathode when brine is electrolysed?

4. How many water molecules does one unit of plaster of Paris hold?