Question

an aqueous solution of copper sulphate was left out in the hot sun. d heat caused some of the d water to evaporate fr d solution. d original 150g solution had a concentration of 10% by mass. if d remaining solution has a concentration of 15% by mass, wat volume volume of water evaporated?
1) 10 ml 2) 15 ml
3) 50 ml 4) 100 ml

Answer 1

Initially  x gm of copper sulphate and 150 grams of solution are there.
      x / 150 = 10 %
      x = 150 * 10 /100 = 15 gm
    Hence, original amount of water in the solution = 150 - 15 = 135 gms

After some water say y gms evaporated. So only (135 - y) gms os remaining.  But copper sulphate remains at 15 gms in the solution.
 
      15 / (15 + 135 - y)  =  15% = 15 /100
           150 - y = 100        =>    y = 50 gms

   hence 50 ml of water has evaporated, as the density of water is 1 gm/ml.