Chemistry problem I am stuck! Please help!

Question

If .240 g of an unknown triprotic acid (H3X) requires 18.00 mL of a .500 M NaOH solution for neutralization, what is the molecular weight of the acid? If it is a common phosphorous containing oxy-acid, what is the formula and name?

 

I think I have a balanced chemical equation of H3X+3NaOH=Na3X+3H2O but I have no idea what to do next please help.

Answer 1

The concepts use to solve this problem are shown below, but the actual answer is not clear based on the results.

 

Your chemical equation shows that the mole ratio of the reactants is

1 H₃X to 3NaOH.

You have reacted 18 mL of .500M NaOH which is

18/1000 L x .500mol/L = .009mol of OH^-

That would indicate that the acid needs to contain .009mol of H⁺

Since the acid is triprotic, there will be .009/3 = .003 mol of the acid.

That means .240g of the acid is .003 mol, so its molar mass is .240g/.003mol or 80g/mol.

Using the molar mass numbers for some possible acids leads to

H₃PO₃ (phosphorous acid) with a molar mass of 82g/mol.

That matches the implied answer in the last part of the problem.

 

HERE IS THE PROBLEM - H₃PO₃ is NOT triprotic! It is diprotic.

I haven't found a triprotic acid with a molar mass close to 80g/mol

H₃PO₄ is triprotic, but its molar mass is 98g/mol which is too much.

I hope another tutor will find a better match to give a better answer.

Answer 2

Your balanced equation looks good. There are 18.00 mL of 0.500 M NaOH.

1. Convert 18.00 mL to L - 0.01800 L

2. Concentration x volume = # moles - 0.500 M x 0.01800 L = 0.00900 moles of NaOH

3. Stoichiomentry - 3 moles NaOH per 1 mole of H3X - 0.00900 / 3 = 0.00300 moles H3X

4. Molar mass (better term than molecular weight) - 0.240 g / 0.00300 moles = 80 g / mol

 

Molar mass is closest to phosphorous acid (H3PO3 = 82 g/mol), but that is diprotic.