91. Calculate the concentration of copper (in mol / l)

The seller is blocked and sales of the product are suspended

91. Calculate the concentration of copper (in mol / l) in the solution if, when analyzing the test solution (20 ml) by the method of additives, a wave of 15 mm was obtained, and after adding 3 ml of a standard solution with a concentration of 0.1 mol / l, the wave height increased to 17.5 mm.