For each of the following solutions, calculate the initial pH and the final pH a
ID: 901982 • Letter: F
Question
For each of the following solutions, calculate the initial pH and the final pH after adding 0.0100 mol of NaOH.
C. For 260.0 mL of a buffer solution that is 0.2551 M in CH3CH2NH2 and 0.2306 M in CH3CH2NH3Cl, calculate the initial pH and the final pH after adding 0.0100 mol of NaOH(Kb=6.46104).
Express your answers using three significant figures separated by a comma.
This is a three part problem I have done the first 2 just fine, but I cannot figure out this one. It kept saying
"Term 2: Not quite. Check through your calculations; you may have made a rounding error or used the wrong number of significant figures."
My answers were: 10.85,11.05 10.85, 10.99 10.85,11.0
Explanation / Answer
Initial pH
Using Hendersen-Hasselbalck equation,
pH = pKa + log[base]/[acid]
pKb = -log[Kb] = 3.19
pKa = 10.81
Feed initial molarity values,
pH = 10.81 + log[0.2551/0.2306]
= 10.844
Final pH
initial moles of CH3CH2NH2 = molarity volume = 0.2551 x 0.260 = 0.066 mols
initial moles of CH3CH2NH3Cl = 0.2306 x 0.260 = 0.060 mols
moles of NaOH added = 0.0100 mols
new moles of CH3CH2NH2 = 0.066 + 0.0100 = 0.0760 mols
new moles of CH3CH2NH3Cl = 0.060 - 0.0100 = 0.0500 mols
molarity of CH3CH2NH2 = 0.0760/0.260 = 0.2923 M
new moles of CH3CH2NH3Cl = 0.0500/0.260 = 0.1923 M
Feed values in Hendersen-Hasselbalck equation,
pH = 10.81 + log[0.2923/0.1923]
= 10.992
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