Question

A 20.00-mL sample of formic acid (HCO2H) is titrated with a 0.100 M solution of NaOH. To reach the endpoint of the titration, 30.00 mL of NaOH solution is required. Ka = 1.8 x 10^-4

What is the concentration of formic acid in the original solution?

What is the pH of the formic acid solution before the titration begins (before the addition of any NaOH)?

Answer 1

You use 30 mL of base (0.1M) to neutrilize the acid.

You use 0.03L*0.1mol/L=0.003mol of OH

since you need 1 mol of base to neutrilize 1 mol of acid you had originally 0.003 moles of formic acid

and the original concentration of the acid using the initial volume is:

Now the pH, you need to write your reaction and the mass action equation:

to calculate the pH in the original solution you need to analize the solution at initial and equilibrium conditions

Initial

Equilibrium:

and using this values in the mass action equation yields:

<------rearranging this equation yields a second degree equation:

solving with thw quadratic equation yields: