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?
2 answers
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) = ?