1) pH vs volume plots: label axes, endpoints, and approximate pH values

a) Draw a graph showing a strong base being titrated by a strong acid?

b) Draw a graph showing a weak base being titrated by a strong acid?

c) Draw a graph showing a weak acid being titrated by a strong base?

d) Draw a graph showing diprotic character?

e) How could the pKb of the weak base, titrated by the strong acid, be determined from the pH vs. vol plot?

f) Circle the buffer regions

2) Answer the questions that follow with reference to the molecules shown below.

a) Which protons of H6EDTA2+ are still bound at pH between 5 and 6? ______

b) You have a solution of Ni2+ of unknown concentration and solution of Na2[H2EDTA] of known concentration. How could you force a sample of Ni2+ to be completely converted to NiEDTA2- at a pH between 5 and 6?

Ni2+ + H2EDTA2- NiEDTA2- + 2H+

c) The excess acid that is generated by the forced chelation is neutralized with NaOH and the solution is buffered to

pH = 5.5. The indicator xylenol orange is added.

i) Under these new conditions the NiEDTA2- will not dechelate. Why not?

ii) Which protons of xylenol orange will be released at the given pH? ______

iii) What is the color of the solution? ______

d) The solution in part c is titrated with a solution of Zn2+ of known concentration. The xylenol orange endpoint is red.

What is occurring at the endpoint, chemically? Qualitatively, how can you determine the concentration of Ni2+ of the original solution introduced in part b?

3) Last year we could not use the lake water because it contained a detergent. A prepared sample of lake water was not red but blue prior to the addition of the Na2H2EDTA solution. Assuming that there is Ca2+ and Mg2+ in the lake water, explain this phenomenon. Be sure to discuss the role of the detergent.

4) When 100.0 mL of a weak acid, HA, were titrated with 0.09381 M NaOH, 27.63 mL were required to reach the endpoint.

a) Calculate the molarity of HA with respect to significant figures (general chemistry rules).

b) What species is dominant at the endpoint? Calculate its concentration with respect to significant figures.

c) The pH at the endpoint was 10.99. Calculate the pKa of HA with respect to significant figures.

d) What was the pH when only 19.47 mL of NaOH had been added.

6) Ana Light was given the job of determining the molecular weight of an unknown weak acid. You have information, from a trusted source, that the acid is acetic acid (CH3COOH) or chloroacetic acid (ClCH2COOH)

or propanoic acid (CH3CH2COOH). Ana shows you that she first dissolved 2.500 g of acid to 250.00 mL in a volumetric flask. Then she titrated a 50.00 mL aliquot of the acid solution with 14.72 mL of 0.5656 M NaOH. Ana believes that the acid is propanoic acid (CH3CH2COOH). Is she correct? Support your answer with calculations.