Louis Zhang
SCH4UE

Unknown Weak Acid Lab

Observations

Mass of KHP (g, ± 0.01) 0.300.300.30
Volume of NaOH (mL, ± 0.1) 8.17.37.5
Shade of pink StrongFaintFaint

Mass of Unknown Acid (g, ± 0.01) 0.300.300.30
Volume of NaOH (mL, ± 0.1) 9.19.08.6
Shade of pink MediumFaintFaint

pH of solution: 2.78 (30g acid, 50mL water)

Analysis

Concentration of NaOH
    Molar mass of KHP: 204.22 g/mol
    Average Volume NaOH: ((7.28 ± 0.1) + (7.46 ± 0.1)) / 2 = 7.37 ± 0.1 mL
    Moles of NaOH = 0.30 g / 204.22(g/mol)
  
Concentration = 0.30 g / 204.22(g/mol) / 7.37 mL = 0.00019932 mol/mL = 0.19932 mol/L

Molar mass acid = Mass / Moles
    Moles acid = Moles NaOH = (Volume NaOH * Concentration NaOH)
    Average Volume NaOH = ((8.96 ± 0.05) + (8.57 ± 0.05)) / 2 = 8.765 ± 0.05 mL
    Moles acid = (0.008765 L * 0.19932 mol/L) = 0.001747 mol

Molar mass acid = 0.30 g / 0.001747 = 171.719 g/mol

concentration [H+]eq = 10 ^ -2.78 = 0.00166 mol/L

Ka = [H+][A-] / [HA]

[H+] = [A-]

Initial [HA]0 = Moles acid / vol acid = 0.001747 mol / 0.05 L = 0.03494 mol/L
At equilibrum [HA]eq = [HA]0 - [H]eq = 0.03494 - 0.00166 = 0.03328 mol/L

Ka = [H+]eq^2 / [HA]eq

Ka = 0.00166 ^ 2 / 0.03328 = 8.280048e-05
Ka = 8.3 * 10-5

Conclusion & Evaluation

The Ka value for the acid was 8.3 * 10-5, and the molar mass was found to be 171.719 g/mol.

This is very close to the properties of ascorbic acid, with a pKa of 4.10 ( = Ka of 7.94 * 10-5) and molar mass of 176.12 g/mol [1].

One strength of this lab was the standardization of the NaOH with KHP. Since NaOH reacts with moisture in the air, it changes the strength/concentration of the acid. KHP is a very stable acid, so using it to calculate the strength of the NaOH gives more accuracy to the overall procedure.
Several weaknesses are present in the titration procedure. Carbon dioxide may have dissolved into the water during titration; this would have affected the calculated values in either direction depending on whether it occurred during standardization or the unknown-acid titration.
The shade of pink observed was subjective and may have varied between titrations. This would also have changed the calculated values in either direction.
The endpoint of the phenolphthalein may not have been equal to the equivalence point of the acid-base reaction. This would have resulted in over- or under-shooting the equivalence point of the unknown weak acid reaction.

References

[1] National Center for Biotechnology Information. (n.d.) Ascorbic acid. PubChem Database. Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/Ascorbic-acid