Question

Consider the titration of a 34.0 mL sample of 0.170 M HBr with 0.205 M KOH. Determine each of the following:

a. the initial pH Express your answer using three decimal places.

pH = ???

b. the volume of added base required to reach the equivalence point Express your answer in milliliters.

V = ???

c. the pH at 10.6 mL of added base Express your answer using three decimal places.

p H = ???

d. the pH at the equivalence point

Express your answer as a whole number.

p H = ???

e. The pH after adding 5.0 mL of base beyond the equivalence point

Express your answer using two decimal places

p H = ???

Answer 1

(A) the initial pH

pH = - log 0.170 = 0.770

(B)the volume of added base required to reach the equivalence point

The equivalence point is reached when moles KOH = moles HBr.
Moles HBr = 0.0340 L X 0.170 mol/L = 5.78X10^-3 mol HBr = 5.78X10^-3 mol KOH

volume KOH = 5.78X10^-3 mol KOH / 0.205 mol/L

= 0.0281 L = 28.1 mL

(c). the pH at 10.6 mL of added base

moles KOH in 10.6 mL = 0.0106 L X 0.205 mol/L = 2.17X10^-3 mol KOH
Excess moles HBr = 5.78X10^-3 mol - 2.17X10^-3 mol = 3.61X10^-3 mol

total volume 44.6 ml
[HBr] = 3.61X10^-3 mol / 0.0446 L = 0.0809 M = [H+]

pH = - log 0.0809
pH = 1.09

(d). the pH at the equivalence point

Since the acid and the base are both strong, the pH at the equivalence point will be 7.00

(e). The pH after adding 5.0 mL of base beyond the equivalence point

Moles KOH in 5 mL = 0.005 L X 0.205 mol/L = 1.02X10^-3 mol
Final volume = 34.0 + 28.1 + 5 = 67.1 mL = 0.0671 L
[OH-] = 1.02X10^-3 mol / 0.0671 L = 0.0152 M
pOH = 1.81
pH = 12.2