Stoichiometry : dilution questions
I gotchu.

It's definitely a dilution question. Just plug it into the equation you have, C1V1=C2V2.

$C_1V_1=C_2V_2$

$(0.2\: M)(500\: mL)=(18\: M)(x\: mL)$

Solve for x:

$x=5.56\: mL$

Therefore, you want to use 5.56 mL of 18 M stock solution. To make your 500 mL of 0.2 M sulfuric acid, just take the 5.56 mL of the stock solution, then fill it up with water until you have 500 mL total solution.

The idea behind the equation, by the way, is to preserve the number of moles in the solution while changing the total volume of the solution. Here, we find out that (0.2 M)(0.5 L) = 0.1 mol of sulfuric acid is needed in the final solution. So, we find the amount of stock solution needed to provide that 0.1 mol of sulfuric acid, by dividing 0.1 mol by the concentration of the stock solution.