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A student is asked to standardize a solution of barium hydroxide. He weighs out 0.945 g potassium hydrogen phthalate (KHC8H4O4, treat this as a monoprotic acid).

It requires 33.9 mL of barium hydroxide to reach the endpoint.

A. What is the molarity of the barium hydroxide solution?

This barium hydroxide solution is then used to titrate an unknown solution of nitric acid.

B. If 19.8 mL of the barium hydroxide solution is required to neutralize 20.4 mL of nitric acid, what is the molarity of the nitric acid solution.

I have no idea how to do this. Thank you for your help!

These are essentially 4-step problems.
1. Write and balance the equation. If we call potassium hydrogen phthalate just KHP, then
Ba(OH)2 + 2KHP ==> 2H2O + BaP + K2P

2. Convert grams to moles. moles KHP = 0.945/molar mass KHC8H4O4 = 0.945/204 = approximately 0.0046

3. Using the coefficients in the balanced equation, convert moles KHP to moles Ba(OH)2
0.0046moles KHP x (1 mole Ba(OH)2/2 moles KHP) = 0.0046 x (1/2) = 0.0023 moles Ba(OH)2.

4. M Ba(OH)2 = moles Ba(OH)2/L Ba(OH)2
M Ba(OH)2 = 0.0023/0.0339 = approximately 0.068 M

Part B is done the same way. Now that you have the Ba(OH)2 standardized, use that, with the same process, to determine the molarity of the HNO3.
Remember that moles = M x L.