Question

20 g of a magnesium carbonate sample decomposes on heating to give carbon dioxide

Answer 1

MgCO₃ ⇒ MgO + CO₂

84g          40g

y g             8 g (y is variable used)

y=(84 x 8)/40 = 16.8 g

Percentage purity of MgCO₃  = (16.8 x 100)/20=84%

Hope it helps!