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

516 g of gypsum, CaSO₄ · 2H₂O (172 g/mol), is heated to 373 K.

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

$516\ \text{g}$ of gypsum, $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$ ( $172\ \text{g/mol}$ ), is heated to $373\ \text{K}$ . Plaster of Paris, $\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O}$ ( $145\ \text{g/mol}$ ), forms. Find the mass of plaster made and the mass of water lost.

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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← Washing soda is Na₂CO₃ · 10H₂O. Use Na = 23, C = 12, O = 16 and H = 1.Say whether each solution is acidic, neutral or basic, and which is the strongest acid. →

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