Question

Consider the following three acids:
​a. Nitric acid trihydrate ( HNO_​3 . 3H_​2O , a monoprotic acid with molar mass = 117.07g/mol )
​b. Oxalic acid ( H_​2C_​2O_​4, a diprotic acid with molar mass = 90.04 g/mol)
​c. Phosphoric acid ( H_​3PO_​4, a triprotic acid with molar mass = 98.00 g/mol)
​A 1.50 g sample of one of the above acids is dissolved in enough water to make a 125.ml acid solution. If 26.40 ml of 1.26 M NaOH is required to titrate the acid solution to completion, what is the identity of the acid? Show all calculations set ups to support your answer for credit.

Answer 1

a. nitric acid is a strong acid that would react with added NaOH

moles HNO3 = 1.50 g/117.07 g/mol = 0.013 mol

moles NaOH added = 1.26 M x 26.40 ml/1000 = 0.033 mol

excess NaOH = 0.02 mol/151.4 ml = 0.000132 M

solution has NaNO2 and NaOH in it.

All of acid is neutralized

b) 1.5 g of oxalic acid

moles of oxalic acid = 1.5/90.04 = 0.017 mol

moles NaOH added = 1.26 M x 26.40 ml/1000 = 0.033 mol

So it would have 0.016 mol of [OOCCOO]^2- and 0.001 mol of HOOCCOO- in it.

c) 1.5 g of H3PO4

moles of H3PO4 = 1.5/98 = 0.018 mol

moles NaOH added = 1.26 M x 26.40 ml/1000 = 0.033 mol

0.015 mol HPO4^2- and 0.003 mol of H2PO4- in solution

So,

a) is basic, b) and c) are buffer solutions