Sodium hydroxide molarity?

Question

Sodium hydroxide solution is usually standardized by titrating a pure sample of potassium hydrogen phthalate (KHP), an acid with one acidic hydrogen and a molar mass of 204.22 g. It takes 31.41 mL of a sodium hydroxide solution to titrate a 0.1167 g sample of KHP. What is the molarity of the sodium hydroxide?

Answer 1

For every mol of KHP added you get one mol of H+ ions.  In order to fully titrate a mixture you will make it completely neutral.  Thus you will eliminate every OH- ion produced by dissolving the NaOH.

Therefore we can find the concentration of NaOH by finding the amount of KHP added in mols, first divided the mass of KHP by it's molecular weight:

n = 0.1167g / 204.22g/mol = 5.71*10^-4 mol

This must be the same amount of OH- ions originally in solution, and since for every OH- ion you have you must have had one NaOH molecule, we know we have 5.71*10^-4 mol oh NaOH.

Now to find concentration, just divide this amount by the original volume:

[NaOH] = n/V = 5.71*10^-4 mol /31.41mL * (1000mL/1L) = 0.018 mol/L

Hope this helped!