Gravimetric Analysis
Review Questions
Problems
4.77 If 30.0 mL of 0.150 M CaCl2 is added to 15.0 mL of 0.100 M AgNO3, what is the mass in grams of AgCl precipitate?
4.78 A sample of 0.6760 g of an unknown compound containing barium ions (Ba2+) is dissolved in water and treated with an excess of Na2SO4. If the mass of the BaSO4 precipitate formed is 0.4105 g, what is the percent by mass of Ba in the original unknown compound?
4.79 How many grams of NaCl are required to precipitate most of the Ag ions from 2.50 X 102 mL of 0.0113 M AgNO3 solution? Write the net ionic equation for the reaction.
4.80 The concentration of Cu2+ ions in the water (which also contains sulfate ions) discharged from a certain industrial plant is determined by adding excess sodium sulfide (Na2S) solution to 0.800 L of the water. The molecular equation is:
Na2S(aq) + CuSO4(aq) → Na2SO4(aq) + CuS(s)
Write the net ionic equation and calculate the molar concentration of Cu2+ in the water sample if 0.0177 g of solid CuS is formed.
Acid-Base Titrations
4.83 A student carried out two titrations using a NaOH solution of unknown concentration in the buret. In one titration she weighed out 0.2458 g of KHP (see p. 153) and transferred it to an Erlenmeyer flask. She then added 20.00 mL of distilled water to dissolve the acid. In the other titration she weighed out 0.2507 g of KHP but added 40.00 mL of distilled water to dissolve the acid. Assuming no experimental error, would she obtain the same result for the concentration of the NaOH solution?
4.84 Would the volume of a 0.10 M NaOH solution needed to titrate 25.0 mL of a 0.10 M HNO2 (a weak acid) solution be different from that needed to titrate 25.0 mL of a 0.10 M HCl (a strong acid) solution?
Problems
4.85 A quantity of 18.68 mL of a KOH solution is needed to neutralize 0.4218 g of KHP. What is the concentration (in molarity) of the KOH solution?
4.86 Calculate the concentration (in molarity) of a NaOH solution if 25.0 mL of the solution are needed to neutralize
17.4 mL of a 0.312 M HCl solution.
4.87 Calculate the volume in mL of a 1.420 M NaOH solution required to titrate the following solutions:
(a) 25.00 mL of a 2.430 M HCl solution
(b) 25.00 mL of a 4.500 M H2SO4 solution
(c) 25.00 mL of a 1.500 M H3PO4 solution
4.88 What volume of a 0.500 M HCl solution is needed to neutralize each of the following:
(a) 10.0 mL of a 0.300 M NaOH solution
(b) 10.0 mL of a 0.200 M Ba(OH) 2 solution
Redox Titrations
Problems
4.91 Iron(II) can be oxidized by an acidic K2Cr2O7 solution according to the net ionic equation:
Cr2O2-7 + 6Fe2+ + 14H→ 2Cr3+ + 6Fe3+ + 7H2O
If it takes 26.0 mL of 0.0250 M K2Cr2O7 to titrate 25.0 mL of a solution containing Fe2+, what is the molar concentration of Fe2+?
4.92 The SO2 present in air is mainly responsible for the acid rain phenomenon. Its concentration can be determined by titrating against a standard permanganate solution as follows:
5SO2 + 2MnO-4 + 2H2O → 5SO2-4 + 2Mn2+ + 4H+
Calculate the number of grams of SO2 in a sample of air if 7.37 mL of 0.00800 M KMnO4 solution are required for the titration.
4.93 A sample of iron ore (containing only Fe2+ ions) weighing 0.2792 g was dissolved in dilute acid solution, and all the Fe(II) was converted to Fe(III) ions. The solution required 23.30 mL of 0.0194 M K2Cr2O7 for titration.
Calculate the percent by mass of iron in the ore. (Hint: See Problem 4.91 for the balanced equation.)
4.94 The concentration of a hydrogen peroxide solution can be conveniently determined by titration against a standardized potassium permanganate solution in an acidic medium according to the following equation:
2MnO-4 + 5H2O2 + 6H+ → 5O2 + 2Mn2+ + 8H2O
If 36.44 mL of a 0.01652 M KMnO4 solution are required to oxidize 25.00 mL of a H2O2 solution, calculate the molarity of the H2O2 solution.
4.95 Oxalic acid (H2C2O4) is present in many plants and vegetables. If 24.0 mL of 0.0100 M KMnO4 solution is needed to titrate 1.00 g of a sample of H2C2O4 to the equivalence point, what is the percent by mass of H2C2O4 in the sample? The net ionic equation is
2MnO-4 + 16H+ + 5C2O2-4 → 2Mn2+ + 10CO2 + 8H2O
4.96 A 15.0-mL sample of an oxalic acid solution requires 25.2 mL of 0.149 M NaOH neutralization. Calculate the volume of a 0.122 M KMnO4 solution needed to react with a second 15.0-mL sample of the oxalic acid solution. (Hint: Oxalic acid is a diprotic acid. See Problem 4.95 for redox equation.)
4.97 Iodate ion, IO-3, oxidizes SO2-3 in acidic solution. The half-reaction for the oxidation is
SO2-3 + H2O → SO2-4 + 2H+ + 2e-
A 100.0-mL sample of solution containing 1.390 g of KIO3 reacts with 32.5 mL of 0.500 M Na2SO3. What is the final oxidation state of the iodine after the reaction has occurred?