CHE 230
Homework Week #7
Directions: You must show all work and report results with the correct number of significant figures.
Name ___________________________
1. The concentration of arsenic in an insecticide is determined gravimetrically by precipitating MgNH4AsO4 and
isolating Mg2As2O7. Determine the %w/w As2O3 in a 1.627-g sample of insecticide if it yields 106.5 mg of
Mg2As2O7.
2. The water content of an 875.4-mg sample of cheese was determined with a moisture analyzer. What is the
%w/w H2O in the cheese if the final mass was found to be 545.8 mg?
3. The fat content of potato chips is determined indirectly by weighing a sample before and after extracting the
fat with supercritical CO2. The following data were obtained for the analysis of potato chips.16
Sample
Number
1
2
3
4
5
Initial Mass (g)
Final Mass (g)
1.1661
1.1723
1.2525
1.2280
1.2837
0.9253
0.9252
0.9850
0.9562
1.0119
(a) Determine the mean and standard deviation for the %w/w fat.
(b) This sample of potato chips is known to have a fat content of 22.7% w/w. Is there any evidence for a
determinate error at a = 0.05?
4. Calculate the solubility (in g per 100 mL) of Mg(OH)2 in unbuffered deionized water. Now calculate the
solubility (in g per 100 mL) of Mg(OH)2 in a solution buffered to a pH of 7.00. How does the buffer affect
solubility? Be sure to state and justify any assumptions you make in solving the problem.
5. The following data for the titration of a monoprotic weak acid with a strong base were collected using an
automatic titrator. Prepare normal, first derivative, and second derivative titration curves for this data. Locate the
equivalence point on each plot. Calculate the ka and concentration of the unknown acid. (Pg 431-432 in the book
may be useful).