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 certain weak acid, HA, with a Ka value of 5.61×106, is titrated with NaOH.

Part A

A solution is made by titrating 9.00 mmol(millimoles) of HA and 1.00 mmol of the strong base. What is the resulting pH?

Express the pH numerically to two decimal places.

Explanation / Answer

HA + OH- ---> A- + H2O
mmoles HA = 9.00 - 1.00 = 8.00
mmoles A- = 1.00

Ka =5.61×106 then pKa = 5.25

pH = 5.25 + log 1.00/ 8.00= 4.35

at the equivalence point [A-] = 0.00900 mol/ 0.0370 L =0.243 M

A- + H2O <----> HA + OH-
K = Kw/Ka = x^2/0.243-x

x = [OH-]= 2.08 x 10^-5 M
pOH= -log[OH-]= pOH = 4.68

pH = 14-pOH
pH = 9.32

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