Question

3.40 g of $CaS{O}_4(MW=136g/mol)$ is formed when 4.30 g of hydrated calcium sulphate heated to a constant mass. How many moles of water of crystallisation are combined with each mole of calcium sulphate?

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

The correct option is B 2

$CaS{O}_4=\frac{3.40}{136}=0.025mole$

Mass of $H_{2}O=$ mass of hydrated salt - mass of anhydrous salt
$=4.30-3.40$

$=0.90g$

$n_{H_{2}O}=\frac{0-90}{18}=0.05mole$

$0.025moleCaS{O}_4\to 0.05mole{H}_{2}O$

$\therefore 1moleCaS{O}_4\to \frac{0.05}{0.025}=2mole{H}_{2}O$

Formula of hydrated $CaS{O}_4$ is $CaS{O}_4.2H_{2}O$.