Chem 2113
Test 2
Fall 1999
Questions are worth 25 points each. OMIT ONE QUESTION by clearly writing OMIT in the space provided for your work. If you
fail to mark OMIT on a question I will omit the last question of the test. Show your work and circle your answers for full credit.
1a. Consider the dissociation of the weak acid acetic acid (Ka = 1.8 x 10-5.)
Calculate the [H3O+] assuming that all activity coefficients are equal to 1.
Calculate the [H3O+] if the acid is dissolved in a solution of 0.050 M NaCl.
Calculate the [H3O+] if the acid is dissolved in a solution of 0.025 M FeSO4.
1b. Calculate the activity of cerium and nitrate in a solution that is 0.0030 M Ce(NO3)3.
2. Calculate the fractions and concentrations for all forms of the weak diprotic acid malic acid when a 0.020M solution is adjusted to
a p[H3O+] of 4.85. (For malic acid, K1 = 3.48 x 10-4 and K2 = 8.00 x 10-6)
3. Sketch the log plot for 0.010 M nitrous acid (Ka = 7.1 x 10-4). and use this information to sketch the titration curve for nitrous acid
with NaOH, labeling the approximate pH values for the initial pH, and the pH at all inflection points (both the buffer region and the
equivalence point). Be sure to label all axes.
4. What is the composition and p[H3O+] of the optimum buffer composed of NH4+ and NH3. at a total concentration of 0.10M. The Ka
for NH4+ is 5.70 x 10-10.
Calculate the number of moles of acid a 100 mL solution of this buffer can withstand before the pH changes by 0.20 units.
5. Consider a mixture that may contain NaOH (39.997), Na2CO3 (105.989) and NaHCO3 (84.007). a 5.000 g sample of an unknown
was dissolved and diluted to 250.0 mL. Two 50.00 mL aliquots of this solution were titrated with 0.1102M HCl, the first to a
phenolphthalein endpoint (32.15 mL) and the second to a methyl red endpoint (47.22 mL). Determine the identity and calculate the
percentage by weight of each species present.
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