Solving a problem in chemistry section 10 point 31

31. The sample is a mixture of potassium chlorate and potassium dichromate weight 19.50 g calcined and got 0.672 liters (eg. In.) Of gas. Another sample of the starting mixture was treated with the same weight of concentrated hydrochloric acid and 6.72 L received (eg. In.) Of gas. Determine the weight fraction of impurities in the starting mixture.