Question

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. Ca...

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 0.070 mol of NaOH to the original buffer.

Express your answer using two decimal places.

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Answer #1

mol of NaOH added = 0.07 mmol

HC2H3O2 will react with OH- to form C2H3O2-

Before Reaction:

mol of C2H3O2- = 0.1 mmol

mol of HC2H3O2 = 0.1 mmol

after reaction,

mol of C2H3O2- = mol present initially + mol added

mol of C2H3O2- = (0.1 + 0.07) mmol

mol of C2H3O2- = 0.17 mmol

mol of HC2H3O2 = mol present initially - mol added

mol of HC2H3O2 = (0.1 - 0.07) mmol

mol of HC2H3O2 = 0.03 mmol

Ka = 1.8*10^-5

pKa = - log (Ka)

= - log(1.8*10^-5)

= 4.745

since volume is both in numerator and denominator, we can use mol instead of concentration

use:

pH = pKa + log {[conjugate base]/[acid]}

= 4.745+ log {0.17/3*10^-2}

= 5.498

Answer: 5.50

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