Question

A) A 75.0mL- volume of 0.200M NH3 (Kb= 1.8*10^-50 is titrated with 0.500HNO3. Calculate the pH after the addition of 1...

A)
A 75.0mL- volume of 0.200M NH3 (Kb= 1.8*10^-50 is titrated with 0.500HNO3. Calculate the pH after the addition of 17.0mL of HNO3 .

B)
A 52.0mL- volume of 0.35M CH3COOH(Ka-1.8*10^-5) is titrated with 0.40M NaOH. Calculate the pH after the addition of 33.0mL of NaOH.

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Answer #1

(A) Initial moles of NH3 = 75/1000 x 0.200 = 0.015 mol

Moles of HNO3 added = 17/1000 x 0.500 = 0.0085 mol

NH3 + HNO3 => NH4+ + NO3-

Moles of NH3 left = 0.015 - 0.0085 = 0.0065 mol

Moles of NH4+ = 0.0085 mol

Ka(NH4+) = Kw/Kb(NH3)

= 10-14/1.8 x 10-5 = 5.556 x 10-10

Henderson-Hasselbalch equation:

pH = pKa + log([NH3]/[NH4+])

= -log Ka + log(moles of NH3/moles of NH4+) since volume is the same for both

= -log(5.556 x 10-10) + log(0.0065/0.0085)

= 9.14

(B) Initial moles of CH3COOH = 52/1000 x 0.35 = 0.0182 mol

Moles of NaOH added = 33/1000 x 0.400 = 0.0132 mol

CH3COOH + NaOH => CH3COO- + Na+

Moles of CH3COOH left = 0.0182 - 0.0132 = 0.005 mol

Moles of CH3COO- = 0.0132 mol

Henderson-Hasselbalch equation:

pH = pKa + log([CH3COO-]/[CH3COOH])

= -log Ka + log(moles of CH3COO-/moles of CH3COOH) since volume is the same for both

= -log(1.8 x 10-5) + log(0.0132/0.005)

= 5.17

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