How many moles of CaCl2 are in a solution of prepared by diluting 250 mL of a 3.0 M of CaCl2 to 750 mL?

Question

I tried to use the fomrula M1V1=M2V2. but still had a hard time getting to the answer of 0.75 mole.

Answer 1

You can't really use M1V1 = M2V2 to find MOLES.  You can use that formula to find moles/liter, or molarity (M).

In this problem, the fact that the original solution is diluted doesn't really matter.  The number of moles of CaCl₂ is determined only by the volume of the 3.0 M solution that is used.  Whether it is diluted to 750 ml or 7500 ml, doesn't change the number of moles in the final solution.  Thus....

250 ml = 0.250 L

0.250 L x 3.0 moles/L = 0.75 moles CaCl₂

Now, if you wanted to solve for the MOLARITY of the final solution, then you could use M1V1 = M2V2, and that answer would be (250 ml)(3.0 M) = (750 ml)(x M) and x = 1.0 M