Chemistry Lab 152
Professor: James Giles
November 7, 2012
The purpose of this experiment was to determine the pKa, Ka, and molar mass of an unknown acid (#14). The pKa was found to be 3.88, the Ka was found to be 1.318 x 10 -4, and the molar mass was found to be 171.9 g/mol.
Acids differ considerable as to their strength. The difference between weak and strong acids can be as much as 10 orders of magnitude. Strong acids dissociate more completely than weak acids, meaning they produce higher concentrations of the conjugate base anion (A-) and the hydronium cation (H30+) in solution.
HA(aq) + H20 (( A- + H3O+
With the following formula the degree to which an acid dissociates (Ka) can be calculated and given a numerical value.
Ka = [A-][H3O+] / [HA]
Ka is the conventional way of measuring an acid’s strength. The purpose of this experiment was to determine the Ka of an unknown acid, along with its pKa and molar mass.
The unknown acid for this experiment was #14.
The experiment began with the preparation and standardization of NaOH solution. It was calculated that 2.00 grams of NaOH pellets were needed to prepare 0.5 L of 0.1 M NaOH solution. The solution was then standardized by conducting three titration trials. It was calculated that 0.7148 grams of KHP were necessary to neutralize 35 mL of the 0.1 M NaOH. Three samples of KHP were weighed approximating this number (Table 1). Each sample was mixed with 40 mL of deionized water and 2 drops of phenolphthalein in 3 Erlenmeyer flasks. Each flask was then titrated with the NaOH to a light pink endpoint. The volumes of NaOH were recorded, averaged, and the standardized. The molarity of the NaOH was found to be 0.0981.
Assuming a molar mass of 100 g/mol, it was calculated that 0.3930 g of acid was needed to neutralize 40 mL
References: Darrell D. Ebbing and Steven D Gammon, General Chemistry, 9th ed. Cengage Learning: Ohio, 2009. Department of Physical Science—Chemistry, Mesa Community College. The Ka and Molar Mass of a Monoprotic Weak Acid (handout).