Question

A solution of HClO₄ was standardized by dissolving 0.4008 g of primary-standard-grade HgO in a solution of KBr:
HgO(s) + 4Br⁻ + H₂O → HgBr₄²⁻ + 2OH⁻
The liberated OH⁻ consumed 43.75 mL of the acid.
Calculate the molar concentration of the HClO₄.

Answer 1

From the equation you know:
1 mol HgO will produce 2 mol OH-
Molar mass HgO = 216.6 g/mol
Mol HgO in 0.4008g = 0.4008/216.6 = 0.00185 mol HgO

This will produce 0.00185 * 2 = 0.00370 mol OH-
HClO4 is a monoprotic acid , therefore
0.00370mol OH- will react with 0.00370 mol acid

43.75 mL of acid solution contains 0.00370 mol of acid

1000mL of acid solution contains : 1000 / 43.75 * 0.00370 = 0.08459 mol of acid

Molarity of HClO4 solution = 0.08459M ( 4 sig figs)

Hope this helps