Question

How many milliliters (ml) of 5 M MgCl2 must be added to 25 ml of a 0.5 M MgCl2 solution
to produce 1500 ml with a final MgCl2 concentration = 1.75 M? How much water?

Answer 1

Let's see how much MgCl2 we need in the final solution:

1500 mL of 1.75 M solution contains
1.5 * 1.75 = 2.625 moles of solute

25 mL of 0.5M solution contains
.025 * 0.5 = .0125 moles of solute

So, we need to add 2.6125 moles of solute.
That means we need 2.6125/5 = .5225 L of 5M solution (522.5 mL)

So, now we have .025+.5225 = .5475L of solution. We want 1.5L, so we have to add

1.500-.5475 = .9525L or 952.5 mL of water