You are working in the laboratory and you need to make a formic acid solution of

Question

You are working in the laboratory and you need to make a formic acid solution of a certain concentration. The lab is out of formic acid (HCOOH), but you find a formic acid solution in the laboratory refrigerator with no concentration label, but you measure the pH and find that it has a pH of 2.8. You know that the pKa for oxalic acid is 3.75. Knowing this information calculate the concentration of the oxalic acid in solution in (mol/L) and in (mg/L) assuming that equilibrium conditions exist between the formic acid and water in solution.

HCOOH <-------> HCOO^- + H^+

Explanation / Answer

Here we have to calculate concentration of oxalic acid

pKa of oxalic acid 3.75

-log (Ka) = 3.75               Ka = -antilog(3.75) = 0.235

pH of formic acid 2.8

-log (H) = 2.8               H = -antilog(2.8) = 0.06

Oxalic acid = HOOC-COOH

Oxalate ion = (C2O2O2)2-

HOOCCOOH ---> (C2O2O2)2- + 2H+

Ka = 0.235 = [Concentrationof COO- ][Concentration of H+]2

0.235 = [Concentrationof COO- ]*(0.06)2

Concentration of oxalate anion = 65.27 Molar =65.27 Moles per litre

65.27*126 = 8224g/litre

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