Question

How would I calculate the molarity of the resulting solution if 25.0mL of 1.50 M HCL solution is diluted to 500 mL. I know you have to change the milliliters to liters by dividing by 1000 and I know that M= Moles of Solute/Liters of Solute but after that I'm lost. Please help!

Answer 1

There are several ways to do it. The easiest is
mL1 x M1 = mL2 x M2
25 x 1.50 = 500 x M2 and solve for M2.

Another is to say the 1.50 M is diluted by a fraction of (25/500) so
1.50 x (25/500) = ??

A more esoteric way is the way you started.
How many moles do you have? That is M x L = 1.50 x 0.025 = 0.0375 moles.
Of course you will have the same moles in the dilute solution; it's just in a larger volume. So the new concn is M = moles/L = 0.0375/0.500 = 0.075 M.
You may want to work through the other methods and see that you get the same value no matter how you do it. Personally, I think changing from mL to L and back to mL is a lot of extra work. I prefer to remember that if M = moles/L, then M = millimoles/mL. Then, for example, the last method is
How many millimoles do we have? That is mL x M = 25 x 1.50 = 37.5 mmoles.
And the new concn will be mmoles/mL = 37.5/500 = 0.075M
Hope this helps although I know you didn't ask for this much detail.