Chemistry: Problem Set

1. a. 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. How many grams of solid H₂SiF₆ should be added to the same reservoir to give 1.2 ppm F^-?

2. a.) 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.) How much would the formality (F) calculated above decrease if the acid were only 7.3% dissociated?

3. 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?

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):

a.) According to the chemist, what is the average lead concentration for the twelve trials?

b.) What is the standard deviation of the measurements?

c.) Is the mean for the twelve trials significantly different from the accepted value at the 95% confidence level?

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

5. a.) How many grams of Ag₂CO₃ (K_sp = 8.1 x 10^-12) can be dissolved in 125 mL of water?

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

c.) 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.

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

b. What is the activity coefficient of Pb²⁺ in this solution?

c. If the activity coefficient of IO₃^- is assumed to be 0.81, what is the solubility of Pb(IO₃)₂ in this solution?

7. 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.) The KHP was dried prior to weighing.

b.) The balance was not properly zeroed, so it always reads exactly 1.00 mg too low.

c.) 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.

Table I. Critical values for the Rejection Quotient, Q