Question

In: Chemistry

1. Suppose you prepare a buffer by dissolving 0.10 mole per liter of a weak acid,...

1. Suppose you prepare a buffer by dissolving 0.10 mole per liter of a weak acid, HA and 0.10 mole per liter of its sodium salt, A-. Assume that pKa = 3. (a) What is the pH of the buffer? How much does the buffer have to be diluted for the pH to increase by 1 unit? Neglect changes in activity coefficients.

Expert Solution

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:

The Weak acid equilibrium:

HA(aq) <-> H+(aq) + A-(aq)

Weak acid = HA(aq)

Conjugate base = A-(aq)

Neutralization of H+ ions:

A-(aq) + H+(aq) <-> HA(aq); in this case, HA is formed, H+ is neutralized as well as A-, the conjugate

Neutralization of OH- ions:

HA(aq) + OH-(aq) <-> H2O(l) + A-(aq) ; in this case; A- is formed, OH- is neutralized as well as HA.

Now,

the buffer equation (Henderson HAsselbach equation)

pH = pKa + log(A-/HA)

initially

pH = 3 + log(0.1/0.1)

pH = 3

b)

find buffer dilution so pH decreases by 1 unit

if we neglect activity

there is no dilution which will change the pH

since

A-/HA ratio is KEPT

therefore, if we increase volume

mathematically, the volumes cancel each other

log(A-/HA)

log( 0.1 mol of A- / V / 0.1 mol of HA / V)

log( 0.1 mol of A- / 0.1 mol of HA )

queen_honey_blossom answered 2 years ago

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