If a crop of wheat removes 20 kg of nitrogen per hectare of soil, what mass of the fertilizer calcium nitrate, Ca(NO₃)₂ would be required to replace nitrogen in 10 hectare field? (N = 14, O = 16, Ca = 40).

According to the question, 20 kg of nitrogen is needed to recover one hectare of soil.
Amount of nitrogen needed to recover 10 hectare of soil = 200 kg
Now,
Molar mass of calcium nitrate = [40 + 2(14 + 48)] g = 164 g
Mass of nitrogen in 1 mole of calcium nitrate = 2(14) g = 28 g
Hence, 28 g of nitrogen is present in 164 g of calcium nitrate.
Amount of calcium nitrate containing 200 kg of nitrogen = $\frac{164}{28}\times 200=1171.4\mathrm{kg}$
Hence, 1171.4 kg of calcium nitrate is needed to recover 10 hectare of soil.