ok help with this chem question?

Question

i hav no idea what to do in this problem!!!!! can anyone explain it to me? here it is:

If you dilute 20.0 mL of a 3.5 M solution to make 231.6 mL of solution, what is the molarity of the dilute solution?

this one is also giving me alot of trouble:

How many milliliters of a 5.0 M H2SO4 stock solution would you need to prepare 126.5 mL of 0.49 M H2SO4?

if u can explain how to solve these problems it would help alot..thanx

Answer 1

1.) Molarity = moles/ liter
So, 20 mL of 3.5 M solution has
3.5 moles/L * .02 L = 0.07 moles

And .07 moles in 231.6 mL has a molarity of:
.07 moles / .2316 L = 0.30M (solution!)

2.) Again, find the # of moles.
.49 moles/L * .1265 L = 0.061985 moles
0.061985 moles / 5 moles/L = 0.012 mL (solution!)