Question

Cyanic Acid is a weak acid

HOCN + H2O <--> H3O+ +OCN- ka= 3.5*10^-4

a) if 2.5ml of 0.01 M cyanic acid solution is added to 25.0 ml of a formis acid buffer with ph = 3.70, what is the ratio of [OCN-]/[HOCN] in the resulting solution?

b) determine the pH of a 0.500 M solution of cyanic acid

c) calculate the pH of a solution that results from titrating 0.0500 moles of cyanic acid to the equivalence point with an equal number of moles of OH-, if the final volume of the solution is 150.0 mL

Answer 1

HOCN ==> H^+ + OCN^-
Ka = (H^+)(OCN^-)/(HOCN)
For a.
The formic acid buffer is pH 3.70 which is H^+ = 2E-4
Ka = (H^+)(OCN^-)/(HOCN)
Rearrange to
(OCN^-)/(HOCN) = Ka/(H^+)
Substitute Ka/(H^+) and you have it.

........HOCN ==> H^+ + OCN^-
initial..0.5......0......0
change.....-x.....x......x
equil....0-.5-x.....x.....x

Ka = (H^+)(OCN^-)/(HOCN)
Substitute and solve for H^+ and convert to pH.

c. The pH is determined by the hydrolysis of the salt at the equivalence point. The (salt) = 0.05mols/0.150L = ?
........OCN^- + HOH ==> HOCN + OH^-
initial..?...............0......0
change...-x..............x......x
equil....?-x..............x.....x

Kb for OCN^- = (Kw/Ka for HOCN) = (HOCN)(OH^-)/(OCN^-)
Substitute from the ICE chart and solve for x = (OH^-) and convert to pH. It will be basic.