A titration involves adding a reactant of known quantity to a solution of an ano

Question

A titration involves adding a reactant of known quantity to a solution of an another reactant while monitoring the equilibrium concentrations. This allows one to determine the concentration of the second reactant. The equation for the reaction of a generic weak acid HA with a strong base is

HA(aq)+OH(aq)A(aq)+H2O(l)

A solution is made by titrating 7.00 mmol (millimoles) of HA and 2.00 mmol of the strong base. What is the resulting pH?

More strong base is added until the equivalence point is reached. What is the pH of this solution at the equivalence point if the total volume is 54.0 mL ?

Explanation / Answer

1)

pKa = -logKa = -log (5.61x 10^-6) = 5.25

HA   + OH-   -----------------> A- + H2O

7            2                             0          0 -------------> initial

5           0                              5           5 ----------------> after reaction

pH = pKa + log [A-/HA]

pH = 5.25 + log (2/5)

pH = 4.85

2)

salt only formed at equivalence point . here salt concnetration = millimoles / total volume

[salt] = C = 7 / 54 = 0.1296 M

pH = 7 + 1/2 [pKa + logC]

pH = 7 + 1/2 [5.25 + log 0.1296]

pH = 9.18

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