Here's an example of how to do your calculation:

You have a solution containing 5mL of an unknown concentration of H_{2}SO_{4}. To figure out the concentration, you add 0.1M NaOH until the solution is fully neutralized. You find that it takes 15.8mL of your NaOH solution to neutralize the H_{2}SO_{4}. How many moles of H_{2}SO_{4} were in your unknown solution?

To find the number of moles of NaOH we used, we recall the definition of molarity. Molarity (M) = moles/L. Therefore, to find the number of moles of NaOH in 15.8mL of 0.1M solution of NaOH, we multiply the concentration by the volume:

0.1 moles/L * 0.0158L = 0.00158 moles

So, in our reaction we used 0.00158 moles of NaOH. Now, the reaction which you track is the following:

2NaOH + H_{2}SO_{4} --> 2H_{2}O + Na_{2}SO_{4}

From the balanced chemical reaction, we see that NaOH and H_{2}SO_{4} react at a 2:1 ratio. Therefore, when the reaction is complete, the number of moles of NaOH reacted = (1/2) * the number of moles of H_{2}SO_{4} reacted. So, we must have started with (1/2)* 0.00158 = 0.00079 moles of H_{2}SO_{4}