chem 221

CHEM 221 Practice Exam / Problem Set #1

This assignment should be completed by 8:00 AM on Wednesday, October 5, 2016

(M/W lab section) or 8:00 AM on Thursday, October 6, 2016 (T/R lab sections). Instructions: • You must show all of your work in order to receive full credit. • Point values for each problem are noted in brackets. • If a problem has multiple parts, be certain to answer all parts. • Indicate your final answers in a clear manner (e.g., underline). • When explaining something, do so in a clear and concise manner using full sentences. • Unless otherwise noted, assume that activity coefficients are unity where applicable. • Do your own work. Collaboration with others on these problems in any form

constitutes academic misconduct. • Bring your completed homework set to lab for discussion with your TA. .

1. a. [2.5 pts] Water is fluoridated to prevent tooth decay. How many liters of 1.0 M H2SiF6 should be added to a city reservoir with a diameter of 100 m and a depth of 20 m to give 1.2 ppb F-?

b. [2.5 pts] How many grams of solid H2SiF6 should be added to the same reservoir to give 1.2 ppm F-?

Name _________________________________ TA ___________________________

2. a.) [10 pts] Calculate the formal (F) and molar (M) concentrations of all constituents present in a solution of trichloroacetic acid, Cl3CCOOH prepared by dissolving 2.85 g of the acid in water to yield 100 mL of aqueous solution. Assume that the acid is 73% ionized in the solution.

b.) [5 pts] How much would the formality (F) calculated above decrease if the acid were only 7.3% dissociated?

3. [10 pts] As part of an experiment in UV-visible spectrophotometry, a group of

students measure the absorbance of a solution of the drink “Kool Aid” at a wavelength of 502 nm, obtaining the following values:

0.189, 0.169, 0.187, 0.183, 0.186, 0.182, 0.181, 0.184, 0.181, and 0.177 Should any of these values be rejected as an outlier on the basis of a Q test? Name _________________________________ TA ___________________________

4. To validate a new analytical method involving the determination of toxic contaminants in soils and vegetation collected from the vicinity of lead smelters, a chemist carries out twelve determinations of the lead concentration in a sample of soil known to contain 77.5 ppb lead, obtaining the following results (all in ppb):

77.09 75.37 72.42 76.84 77.84 76.69 78.03 74.96 77.54 76.09 81.12 75.75 a.) [2 pts] According to the chemist, what is the average lead concentration for the

twelve trials? b.) [3 pts] What is the standard deviation of the measurements? c.) [5 pts] Is the mean for the twelve trials significantly different from the accepted

value at the 95% confidence level? Name _____________________________ TA _______________________________

d.) [5 pts] Is there any indication that the new method suffers from determinate error? Explain.

5. a.) [5 pts] How many grams of Ag2CO3 (Ksp = 8.1 x 10-12) can be dissolved in 125

mL of water?

b.) [5 pts] How would the result be affected by the presence in solution of silver ion from another source (e.g., addition of AgNO3)? Explain.

c.) [5 pts] Silver ion is among many metal ions known to form strong, water-soluble complexes with ammonia:

Ag+ + 2 NH3 → Ag(NH3)2+

How would you expect the results of Part A to be affected by the presence in solution of ammonia? Explain.

Name _________________________________ TA ___________________________

6. a. [5 points] What is the ionic strength of a 0.020 M Mg(NO3)2 solution saturated with the slightly soluble salt Pb(IO3)2 (Ksp = 2.5 x 10-13)?

b. [5 points] What is the activity coefficient of Pb2+ in this solution? c. [5 points] If the activity coefficient of IO3- is assumed to be 0.81, what is the

solubility of Pb(IO3)2 in this solution? Name _____________________________ TA ________________________________

7. [10 points] Consider a mixture of the two solids BaCl2 • 2H2O (FW = 244.26 g/mol) and KCl (FW = 74.551 g/mol). When the mixture is heated to 160 ºC for 1 hour, the water of crystallization is driven off:

BaCl2 • 2H2O (s) → BaCl2 (s) + 2 H2O (g) A sample originally weighing 1.7839 g is found to weigh 1.5623 g after heating.

Calculate the weight percent of Ba, K, and Cl in the original sample.

8. An approximately 0.1 M NaOH solution is prepared by adding 2.0 g of solid NaOH to 500 mL of water. The precise concentration of the solution is determined by titrating the NaOH against weighed portions of a weak acid, potassium hydrogen phthalate (KHP), obtained from the National Institute of Standards and Technology (NIST) and certified as being 99.99% pure. The KHP samples are weighed by difference on an analytical balance. Following its standardization, the NaOH solution is used to determine the concentration of acetic acid in a sample of commercial vinegar. How would the calculated acetic acid concentration be affected (in comparison with the true concentration) if each of the following three events were to occur? Explain your answers.

a.) [5 pts] The KHP was dried prior to weighing.

Name _____________________________ TA ________________________________

b.) [5 pts] The balance was not properly zeroed, so it always reads exactly 1.00 mg

too low.

c.) [5 pts] During the standardization of the NaOH solution, when the buret was filled prior to each titration, an air bubble became trapped in the tip of the buret, but it became dislodged during the titration.

Name _____________________________ TA ________________________________

Table I. Critical values for the Rejection Quotient, Q

Number of observations Qcrit (90% confidence) 2 - 3 0.94 4 0.76 5 0.64 6 0.56 7 0.51 8 0.47 9 0.44 10 0.41