Title: Determining the dissociation of a weak acid using pH Measurements Data: M
ID: 875709 • Letter: T
Question
Title: Determining the dissociation of a weak acid using pH Measurements
Data:
Molarity of unknown acid 1.02 M
NaOH solution: 0.484 M
Volume of Unknown acid, mL: 40.0 mL
Final Buret Reading, mL: 40.0 mL
Initial Buret Reading, mL: 0.00 mL
Volume of NaOH solution, mL: 12.50 mL
Final buret reading, mL ; 12.50 mL
Initial buret reading, mL: 0.00 mL
Total Volume of solution, mL: 250.0 mL
pH reading: 4.56
I need to find the following: Calculating Ka of unknown acid
Initial number of moles for HAn(aq) & OH- (aq)
Number of moles at equilibrium: HAn(aq) & An-
Equilibrium concentration, mol L^-: HAn, An- & H3O^+
Ka ?
Explanation / Answer
acid millimole = 1.02 x 40 = 40.8
NaOH millimoles =0.484 x 12.5 = 6.05
NaOH + HA ---------------------> NaA + H2O
6.05 40.8 0 0
0 34.75 6.05 6.05
in the solution HA + NaA remained . it is buffer solution
pH = pKa + log [salt/acid]
pH = pKa + log [NaA/HA]
4.56 = pKa + log (6.05/34.75)
pKa = 5.3
pKa = -logKa
5.3 = -log Ka
Ka = 5.01 x 10^-6
initial number of moles of HAn and OH-
there is no salt formed initially so HAn moles = 0
moles of OH- = 0.484 x 12.5 /250
= 0.0242
Number of moles at equilibrium: HAn(aq) & An-
moles of HAn = millimoles / total volume
= 34.75 / 250
= 0.139
moles of An- = millimoles / total volume
= 6.05 / 250
= 0.0242
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