Question

1. A buffer is made by mixing 1.250 mol formic acid, HCOOH, with 0.750 mol sodium formate, HCOONa in enough water for a total volume of 1.00L. The Ka of formic acid is 1.87 x 10-4

a. What is the pH of this buffer?

b. If 0.075 mol HCl is added to this buffer, what is the pH of the resulting buffer? (assume no volume changes)

c. If 0.055 mol NaOH is added to this buffer, what is the pH of the resulting buffer? (Assume no volume changes)

2. A buffer solution contains 0.120 moles of propionic acid, HC3H5O2, and 0.100 moles of sodium propionate, NaC3H5O2, in 1.00 L of solution. Ka = 1.32 x 10-5 a. What is the pH of this buffer?

b. What is the pH of this buffer after the addition of 0.010 mol of NaOH?

c. Beginning with the initial buffer from part a, what is the pH of the buffer after the addition of 0.010 mol of HBr?

Answer 1

1.

a)

Ka = 1.87 x 10^-4

pKa = -log Ka = 3.728

Moles of formic acid = 1.250

moles of sodium formate = 0.750

pH = pKa + log [salt / acid]

   = 3.728 + log [0.750 / 1.250]

pH = 3.51

b.)

moles of HCl added = 0.075 mol = C

pH = pKa + log [salt - C / acid + C]

    = 3.728 + log [0.750 - 0.075 / 1.250 + 0.075]

pH = 3.44

c)

moels of NaOH added = 0.055 = C

pH = pKa + log [salt + C / acid - C]

    = 3.728 + log [0.750 + 0.055 / 1.250 - 0.055]

pH = 3.56