Question

1. A solution is prepared such that it is 0.45 M in formic acid and 0.35 M in sodium formate.
a) Where is this mixture located on a titration curve: before the buffer point, at the buffer point, or after the
buffer point?
b) Use the Henderson-Hasselbach to determine the pH of the solution.
c) Ignoring volume effects, determine the pH of the solution when 0.07 M NaOH has been added
LR=Limiting reactant
HA + OH <--> A- + H2O
I 0.45 0.35 - -
LR-0.35 -0.35 +0.35 -
I 0.10 - 0.35 -
C +x +x -x -
E 0.10-x x 0.35-x -

K is Huge the x is removed thus the assumption is hat x<<0.10 and x<<0.35

K = [A-]/[HA][OH-]
1.047 x 10^10 = [0.35M]/[0.10M][x]
2.7 x 10^-9 = [OH-]

Because [A-] is greater the mixture is located after the 1/2 equivalence point.

b. pOH = -log(2.7 x 10^-90
pOH = 8.56
pH = 14- 8.56
pH = 5.44
I am not sure if i did this problem correctly can some one please check and
I don't know how to start part c of the problem?

Answer 1

Are you sure about part a? I think an error is starting with 0.45M formic acid. Wouldn't you need to start with more formic acid to end up with 0.45M?

If you used the HH equation for part b you should have come up with 3.64 and you're a long way from that. I used 3.75 for pKa formic acid.

c.
.........HA + OH^- --> A^- + H2O
I.......0.45..0.......0.35......
add..........0.072..............
C......-0.072.-0.072..+0.072
E.......0.378...0.....0.0.422

pH = 3.75 + log(0.422/0.378) = ?

Answer 2

You right the pH for b = 3.64. and for c =3.78 i messed up on equation. Thank you for for help