Question

Buffers (These questions must go together because they are all related to buffers.)

A) Why must the target pH be within 1 point of the pKa and not be able to exceed this value?

B) After the solids were dissolved in the water first, why was the solution then topped up to the final buffer volume?

Answer all questions thouroughly and clearly so that it'd make sense to a person who lack knowledge in chemistry.

Answer 1

Ans:- A buffer solution is a mixture of a weak acid and weak base and its salt i.e conjugate base or conjugate acid respectively that permits the solution to resist the large change in the pH upon addition strong acid and strong base. To have a better clarification we can site a simple example:

HF and NaF Buffer. HF is a weak acid because of the strong attraction between the F^- and H⁺ ion. As aresult it doesnot dissociate completely in water.

HF(aq)+H₂O(l)⇌F⁻(aq)+H₃O⁺(aq)

If we now add NaF to the solution...it will completely dissociate into Na⁺ and F^- ion. Now that F^- will combine will H₃O⁺ and gives back to HF.In order to know how much acid has been dissociated we have an acid dissociation const. Ka.

Ka=[H⁺][F^-]/[HF]as we know that acid is substance that can liberate H⁺ easily, so to determine that valuewe have pKa = - log Ka and pH = - log [H+ ]

Now to know whether the aqueous solution of a conjugate base/conjugate acid is functioning as buffer or not we have another expression that is called Henderson-Hasselbalch equation:

pH=pKa+log [A^-]/[HA]

now if the concentration of conjugate base and weak acid becomes equal...which happens during the pH ,then [A^-]/[HA] becomes 1. and log 1 =0

then

pH=pKa

this is the condition for best buffer.

Now suppose lets say that ratio of the concentration of concentration of conjugate base and weak acid becomes 9:1 or 1:9..that means improper dissociation, we get

pH=pKa+/- 1

that is one point of pKa. and it must be within one point of pKa so that all the H+ of the strong acid must get consumes by the conjugate base so that more weak acid is formed and pH of the soln remains constant.

Ans b:- yes the solid was first dissolved at small amount of water and then topped to the required amount of buffer solution. this is because at the first weak acid is dissociated very less, that means the conjugate base dissociated is very less. and more over the concentration of the solute would be too high. so inorder to maintain the required pH we add that much amount of water that could maintain the required pH. direct dilution would affect the required pH.