Question

5 1 Liter of a Buter solution is О. Іго и HC7H5O2 and 0.250m Nacy HS O2, calcolate the pt of the solution after the addition of 100.0 ml of IM HCl. The ka For Hcy Hj oz is -5 6.5x10

Answer 1

looks like the acid is benzoic acid C6H5COOH

For short - hand I'll call this HA

Benzoic acid dissociates:

HA(aq)⇌H+(aq)+A− (aq)(1)

The lithium salt provides a large reserve of the co - base A− by dissociating completely:

LiA(s)→Li+(aq)+A− (aq)(2)

The initial no. moles of A− is given by:

nA−init=1×0.25=0.25

The no. H+ added is given by:

H+=1×1001000=0.1

From (1) you can see that these H+ ions will be absorbed by the large reserve of A− ions shifting the equilibrium to the left thus "buffering" the pH.

You can see that they react in a 1:1 molar ratio so that the no. of moles of A− remaining is given by:

nA−=0.25−0.1=0.15

Each mole of H+ added will form 1 mole of HA so no. moles HA formed =0.1

The initial moles of HA is given by:

nHAinit=0.15×1=0.15

So the total moles HA is given by:

nHA=0.1+0.15=0.25

The expression for the acid dissociation constant Ka is given by:

Ka=[H+(aq)][A−(aq)][HA(aq)]

Rearranging:

[H+(aq)]=Ka×[HA(aq)][A−(aq)]

I will ignore any changes in concentration due to the dissociation of the acid as it is very small.

The total volume is common to both acid and co - base and will cancel so we can write:

[H+(aq)]=6.5×10−5×0.250.15=1.083×10−4

pH=−log(1.083×10−4)

pH=3.96