Question

A buffer solution contains 0.27 mol of ascorbic acid (HC₆H₇O₆) and 0.76 mol of sodium ascorbate (NaC₆H₇O₆) in 8.20 L.
The K_a of ascorbic acid (HC₆H₇O₆) is K_a = 8e-05.



(a) What is the pH of this buffer?

pH =   


(b) What is the pH of the buffer after the addition of 0.08 mol of NaOH? (assume no volume change)

pH =  


(c) What is the pH of the original buffer after the addition of 0.39 mol of HI? (assume no volume change)

pH =  

Answer 1

a) Use Henderson-Hasselbach equation

pH = pka + Log ([Conjugate base]/[weak acid ] )

[conjugate base] = [ Salt ]

Since volume is same for both, we can use only use mole ratio

Pka = -log ka = -log 8.0 x 10^-5 = 4.1

pH = 4.1+log (0.76/0.27) = 5.55

b) When we add 0.08 mol NaOH, moles of acid = original mol - 0.08 mol

And moles of conjugate base = original mol + 0.08 mol

(mol ratio is 1:1)

Equilibrium moles of acid = 0.27-0.08=0.19 mol acid

Equilibrium moles of base = 0.76+0.08=0.84 mol base

pH= 4.1+log (0.84/0.19)=4.74

c)

When we add acid, equilibrium moles of weak acid = 0.27+0.39=0.66 mol

Equilibrium moles base = 0.76-0.39=0.37 mol base

pH = 4.1+log(0.37/0.66)

pH = 3.85