Sodium nitrate is less soluble in cool water than warm. 144.0 grams are dissolved at 50°C, multiplying by 2 since there are 200 grams H2O = 72.0 grams. Then 88.0 grams are dissolved at 20°C, again multiplying by 2= 44.0 grams. Subtracting both we get the final value that 28.0 grams precipitate from the saturated solution.
