If 100.0 ml of HCI stock solution is used to make 500.0-ml of a 0.675 M HCI dilution, what is the molarity of the stock solution?

3.375 M.

Explanation:

It is known that the no. of millimoles of HCl before dilution = the no. of millimoles of HCl after dilution.

∵ (MV) before dilution = (MV) after dilution.

∴ M before dilution = (MV) after dilution / V before dilution = (0.675 M) (500.0 mL) / (100.0 mL) = 3.375 M.