Question

A solution of formic acid (HCOOH) has a pH of 2.25. How many grams of formic acid are there in 100.0 mL of the solution?

Answer 1

ka of formic acid = 1.8*10^-4

Let the concentration of HCOOH be c

use:

pH = -log [H+]

2.25 = -log [H+]

[H+] = 5.623*10^-3 M

HCOOH dissociates as:

HCOOH -----> H+ + HCOO-

c 0 0

c-x x x

Ka = [H+][HCOO-]/[HCOOH]

Ka = x*x/(c-x)

1.8*10^-4 = 5.623*10^-3*5.623*10^-3/(c-5.623*10^-3)

c-5.623*10^-3 = 0.1757

c=0.1813

volume , V = 1*10^2 mL

= 0.1 L

use:

number of mol,

n = Molarity * Volume

= 0.1813*0.1

= 1.813*10^-2 mol

Molar mass of HCOOH,

MM = 1*MM(C) + 2*MM(O) + 2*MM(H)

= 1*12.01 + 2*16.0 + 2*1.008

= 46.026 g/mol

use:

mass of HCOOH,

m = number of mol * molar mass

= 1.813*10^-2 mol * 46.03 g/mol

= 0.8345 g

Answer: 0.834 g