16.50 For each of the following solutions, calculate the initial pH and the fina
ID: 487410 • Letter: 1
Question
16.50
For each of the following solutions, calculate the initial pH and the final pH after adding 0.0150 mol of NaOH
For 240.0 mL of a buffer solution that is 0.205 M in HCHO2 and 0.300 M in KCHO2, calculate the initial pH and the final pH after adding 0.0150 mol of NaOH(Ka=1.8104).
Express your answers using two decimal places separated by a comma.
For 240.0 mL of a buffer solution that is 0.2922 M in CH3CH2NH2 and 0.2622 M in CH3CH2NH3Cl, calculate the initial pH and the final pH after adding 0.0150 mol of NaOH(Kb=5.6104).
Express your answers using two decimal places separated by a comma.
Explanation / Answer
a) volume of buffer = 240 mL
[acid] = 0.205 M and [conjugate base] = 0.3
Ka of acid = 1.8x10-4
initial pH calculation
pH of a buffer is given by Hendersen equation as
pH = pKa + log [conjugae base]/[acid]
= [4 - log 1.8] + log [0.3/0.205]
= 3.905
Now 0.0150mol of NaOH is added to this solution
the reaction is
HA + OH - ---------> A - + H2O
240x0.205 0 240x0.3 - initial mmoles
15 change
34.2 0 64.2 - mmoles after addition
Thus new pH = 3.74 + log 64.2/34.2
= 5.617
b) [base] = 0.2922 M and [conjugate acid ] = 0.2622M and Kb = 5.6x10-4
Thus pOH = pKb + log [conjugate acid]/[base]
= [4-log 5.6] + log 0.2622/0.2922
= 3.2047
thus pH = 14-3.2047
= 10.795
Now pH after adding 0.015 mol NaOH
BH+ + OH- ----------> B + H2O
240x0.2622 0 240x 0.2922 - initial mmoles
15 change
47.928 0 85.128 mmoles after addition
Thus the pOH = [4-log5.6] + log 47.928/85.128
=3.0024
and the pH = 10.99
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