Question

You want to clean a 500-ml flask that has been used to store a 0.9m solution. each time the flask is emptied, 1.00 ml of solution adheres to the walls, and thus remains in the flask. for each rinse cycle, you pour 9.00 ml of solvent into the flask (to make 10.00 ml total), swirl to mix uniformly, and then empty it. what is the minimum number of such rinses necessary to reduce the residual concentration of 0.0001 m or below?

Answer 1

In this question, every time you rinse the flask, 1 ml of the solution will adhere and you add it with 9ml which will result as 10ml(1ml + 9ml). So, every time you rinse the flask it concentration will drop to 1/10 times of before. If the solution initial concentration is 0.9m, then the amount needed to rinse is:


Final concentration <= initial concentration *  (1/10) ^ rinse 
0.0001m<= 0.9m * 1/10^rinse
0.001m/ 0.9m<= 1/ 10^ rinse
0.11 * 10^-3 <=1* 10 ^ - rinse
rinse= 3

it needs at least 3x rinse to get the concentration into <0.0001 m