MediumChemistryHigh School (9–10)GeneralAcids, Bases and Salts › Salts and Their UsesNumerical

Washing soda is Na₂CO₃ · 10H₂O. Use Na = 23, C = 12, O = 16 and H = 1.

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Washing soda is $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ . Use $\text{Na} = 23$ , $\text{C} = 12$ , $\text{O} = 16$ and $\text{H} = 1$ .

What percentage of its mass is sodium?
What mass of water is there in $57.2\ \text{g}$ of the crystals?

Need a reminder? The key ideaShow

Water of crystallisation is the fixed number of water molecules in a crystal: $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ , $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ , $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$ . Gypsum 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.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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← 9.98 g of blue copper sulphate, CuSO₄ · 5H₂O (249.5 g/mol), is heated until it is white.516 g of gypsum, CaSO₄ · 2H₂O (172 g/mol), is heated to 373 K. →

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