Question

Part A Consider a buffer solution that is 0.50 M in NH3 and 0.20 M in...

Part A

Consider a buffer solution that is 0.50 M in NH3 and 0.20 M in NH4Cl. For ammonia, pKb=4.75.​

Calculate the pH of 1.0 L of the original buffer, upon addition of 0.190 mol of solid NaOH.

Part B

A 1.0-L buffer solution contains 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2. The value of Ka for HC2H3O2 is 1.8×10−5.​

Calculate the pH of the solution, upon addition of 0.030 mol of NaOH to the original buffer.

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Homework Answers

Answer #1

part A)

NH4Cl + NaOH -------------> NH3 + NaCl + H2O

after 0.190/1.0 = 0.190 M NaOH added

[NH3] = 0.50 + 0.190 = 0.690 M

[NH4Cl] = 0.20 - 0.190 = 0.01 M

pOH = pKb + log [NH4Cl] / [NH3]

pOH = 4.75 + log [0.01] / [0.690]

pOH = 2.91

pH = 14 - 2.91

pH = 11.09

part B)

HC2H3O2 + NaOH -----------> NaC2H3O2 + H2O

up on addition of 0.030 / 1.0 = 0.030 M NaOH

[HC2H3O2] = 0.100 - 0.030 = 0.07 M

[NaC2H3O2] = 0.100 + 0.03 = 0.130M

pH = pKb + log [NaC2H3O2]/[HC2H3O2]

pKb = - log Kb = -log [1.8 x 10-5 ] = 4.74

pH = 4.74 + log [0.130] / [0.07]

pH = 5.01

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