Question

What is the difference (in pH units) between the initial pH of a 0.20 M NH3 / 0.20 M NH4Cl buffer to the pH of the buffer after the addition of 10.0 mL of 0.10 M HCl to 65.0 mL of the buffer? (Ka of NH4+ at 25°C = 5.6 x 10−10)

a. final pH is 0.07 pH units lower than original pH

b. there is no measurable difference between final and initial pH - that is why it is called a buffer

c. final pH is 0.16 pH units lower than original pH

d. final pH is 0.16 pH units higher than original pH

e. final pH is 0.07 pH units higher than original pH

Answer 1

A buffer is any type of substance that will resist pH change when H+ or OH- is added.

This is typically achieved with equilibrium equations. Both type of buffer will resist both type of additions.

When a weak acid and its conjugate base are added, they will form a buffer

The equations:

For the weak base equilibrium:

B(aq) + H2O(l) <-> BH+(aq) + OH-(aq)

Weak base = B;

Conjugate acid = BH+

Neutralization of OH- ions:

BH+(aq) + OH-(aq) <-> B(aq) + H2O(l); in this case, OH- is neutralized by BH+, as well as B is created

Neutralization of H+ ions:

B(aq) + H+(aq) <-> BH+(aq)

Then, the henderson hasselbach

pH = pKa + log(NH3/NH4+)

pKa = -log(5.6*10^-10) = 9.25 approx

pH = 9.25 + log(NH3/NH4+)

when adding HCl to th ebuffer... expect

initially

mmol of NH3 = MV = (0.2*65) = 13

mmol of NH4+ = MV = (0.2*65) = 13

after addition of

mmol of H+ = MV = 10*0.1 = 1 mmol

mmol of NH3 = 13 - 1 = 12

mmol of NH4+ = 13 + 1 = 14

then

pH = 9.25 + log(12/14)

pH = 9.25 - 0.07

choose A