Question

Suppose you have a 1.00 L of an aqueous buffer containing 60.0 mmol formic acid (pKa=3.74) and 40.0 mmol formate. Calculate the pH of this buffer.

What volume of 4.00 M NaOH would be required to increase the pH to 4.93?

Answer 1

a)

pH = pKa+ log(formate/formic aicd)

pH = 3.74 + log(40/60)

pH = 3.56390

b)

find V NAOH for pH = 4.93

let x be mmol of OH- ions

mmol of formate formed = 60 + x

mmol of formic acid left = 40 - x

pH = pKa + log(Formate/Formic aicd)

4.93 = 3.74 + log( (60+x) /(40-x))

10^(4.93-3.74) =  ( (60+x) /(40-x))

15.488 = (60+x) /(40-x)

15.488 *40 - 15.488 x = 60+x

15.488 *40 -60 = 16.488x

x = (15.488 *40 -60 ) / (16.488)

x = 33.934

mmo of NaOH = 33.934

V = mmol/M

V = 33.934 / 4

V = 8.4835mL required