[chelsea20]

What is the molarity of a solution of H2SO4 if 100 ml of this solution requires 25 ml of 2.0 M NaOH for complete neutralization?

The answer shows that Molarity= number of moles of NaOH used divided by 2 then divided by 100 ml
My question is why do you divide the number of moles of NaOH by 2? By the way how do you know which one is the solute?

Thank you!!

[Unknown008]

There is no need to find the solute. What you really need, is a balanced equation to help you understand.



Let's take it step by step.
You used 25 mL of 2.0 M NaOH.
How many moles of NaOH is present in 25 mL of 2.0 M of NaOH?

1000 mL -> 2 moles
1 mL -> 2/1000 moles = 0.002 mol
25 mL -> 0.002 * 25 = 0.05 mol

Now, look at your equation. Two moles of NaOH would react with one mole of H2SO4. Since you have used 0.05 mol NaOH, that surely means that there was half that amount of H2SO4! Right? Do you understand?

That means, there was 0.05/2 = 0.0025 mol of H2SO4.

Now, use proportions again.
100 mL -> 0.0025 mol
1 mL -> 0.0025/100 = 0.000025 mol
1000 mL -> 0.000025 * 1000 = 0.025 mol

Since you get 0.025 mol of H2SO4 in 1000 mL, it means that it's concentration (molarity) is 0.025 M.

I hope it helped! :)