Question

Which of the following 1.0 L mixtures has the greatest buffering capacity?

A

0.1 M NH4Cl +0.2 M NH3

B

1.0 M NH4Cl + 0.1 M NH3

C

0.2 M NH4Cl +0.8 M NH3

D

1.0 M NH4Cl + 1.2 M NH3

Arrange the following mixtures of buffer solutions in an increasing order of their pH values.

Drag and drop to order

1

A

0.10 M KF/0.10 M HF

2

B

0.20 M KF/0.10 M HF

3

C

0.10 M KF/0.20 M HF

Answer 1

Solution 1

To find out the capacity of the buffer we will see that the pH value doesnt change with the change in concentration. And the one changing the least would be the one with the best capacity

pH = pKa + log (salt)/(base)

Now we see the ration in the above 4 cases,

And if the value is close to one than it is the best buffer

a) [NH4CL/NH3] = 0.1/0.2 = 0.5

b) [NH4CL/NH3] = 1/0.1 = 10

c) [NH4CL/NH3] = 0.2/0.8 = 0.25

d) [NH4CL/NH3] = 1/1.2 = 0.833

So the option D will be the best buffer capacity

Solution 2

We calculate the buffer capacity in the same way as we calculated above

pH = pKa + log [Salt]/[Acid]

a) [KF/HF] = 0.1/0.1 = 1

b) [KF/HF] = 0.2/0.1 = 2

c) [KF/HF] = 0.1/0.2 = 0.5

Increasing order of pH = C < A < B