Question

The answer is B, but I really need a detailed explanation for this.

Equal volumes of the following pairs of solutions are mixed. Which pair will produce a buffer solution?

A. 0.10 mol L-1 HCl and 0.05 mol L-1 NaOH

B. 0.10 mol L-1 HCl and 0.15 mol L-1 NH3

C. 0.10 mol L-1 HCl and 0.05 mol L-1 NH3

D. 0.10 mol L-1 HCl and 0.20 mol L-1 CH3COOH

E. 0.10 mol L-1 HCl and 0.20 mol L-1 NaCl

Answer 1

A buffer solution is a solution whose pH remains almost same on addtion of a small amount of acid or base and helps in maintaining a constant pH over a long period of time.

A buffer solution is either composed of a weak acid and a salt of its conjugate base or a weak base and a salt of its conjugate acid.

In option A, HCl is a strong acid and NaOH is a strong base. Hence they cannot form a buffer solution.

In option B, NH3 is a weak base and HCl is a strong acid.

Since volume is same for both, for unit volume(say 1L) of each,

mles of HCl = MxV = 0.10 mol

moles of NH3 = MxV = 0.15 mol

Now HCl will react with weak base NH3 to form conjugate base NH₄⁺

HCl + NH3 ---> NH₄⁺ (aq) + Cl^-(aq)

1 mol 1 mol 1 mol

Hence the moles of HCl that will react with the moles of NH3 = 0.10 mol HCl x (1 mol NH3 / 1 mol HCl)

= 0.10 mol NH3.

Hence HCl is the limiting reactant and there would be no more HCl at equilibrium. Also the equilibrium solution would contain 0.05 mol of NH3 (weak base) and 0.10 mol of NH₄⁺ (aq) (conjugate base of weak base).

Hence this solution behave as buffer solution.

In option C, NH3 being present in smaller amount than HCl will be completely neutralized and hence the final solution will only contain strong acid HCl and conjugate base NH4+. This is not a required criteria for the formation of a buffer solution. Hence this solution cannot form a buffer solution.

In option D, there is a strong acid and a salt. Due to absence of a weak acid or base they cannot form a buffer solution.

Hence B is the correct answer.