I think I'm over thinking this

Question

I feel like i'm over thinking this but every time i do it i get it wrong.

Here's the problem

In a titration, 402.8 mL of a 7.16 M solution of hydrochloric acid is neutralized by with 0.359 L of magnesium hydroxide. What is the molarity of the base?

Answer 1

So, write down the balanced equation so you can see what's going on.

2HCl + Mg(OH)₂ ==> 2H₂O + MgCl₂

Now that we see the mol ratio of HCl : Mg(OH)2, we can use this to find moles of Mg(OH)2 that are present:

402.8 ml HCl x 1 L/1000 ml x 7.16 mol / L x 1 mol Mg(OH)₂ / 2 mol HCl = 1.44 mol Mg(OH)₂

This is what's present in the 0.359 L, so concentration is ...

1.44 mol / 0.359 L = 4.02 M (3 sig. figs.)