Chemistry 213
Summer 2010
Assignment No. 2
Name:
__________ ________
Stoichiometry, volumetric and gravimetric analyses
MULTIPLE CHOICE QUESTIONS
Select the one best answer for each question.
1. In the reaction CaCO3 + 2HNO3 → Ca(NO3)2 + CO2 + H2O, how many milliliters of
0.1015 M HNO3 are required to react with 0.9857 grams of CaCO3?
(a) 0.1940 mL
(b) 97.04 mL
(c) 194.1 mL
2. In an experiment, an average of 67.0 ml of 0.468 mol/L magnesium hydroxide
(Mg(OH)2 ) is required to titrate 50.0 ml of the acid in order to reach the endpoint.
The neutralization reaction is:
2CH3COOH (aq) + Mg(OH)2 (aq) → Mg(CH3COO)2 (aq) + 2H2O (l)
The concentration of acetic acid (CH3COOH) is
(a) 0.627 mol/L
(b) 1.25 mol/L
(c) 0.314 mol/L.
3. Which statement concerning titrations is incorrect?
(a) The titration error can be accounted for by running a blank titration.
(b) The titrant is a primary standard after standardization.
(c) The endpoint and equivalence point are not the same.
4. The end point of a titration is defined as
(a) the equivalence point of the titration.
(b) the actual measured volume of titrant required to complete a titration.
(c) the volume associated with actual stoichiometric quantity of titrant required to
complete a titration.
5. Iron in the +2 oxidation state reacts with potassium dichromate to produce Fe3+ and
Cr3+ according to the equation 6Fe2+ + Cr2O72- + 14H+ → 6Fe3+ + 2Cr3+ + 7H2O. How
many milliliters of 0.1658 M K2Cr2O7 are required to titrate 200.0 mL of 0.2500 M
Fe2+ solution?
(a) 50.26 mL
(b) 301.6 mL
(c) 8.290 mL
6. A 0.42 gram sample of acetylsalicylic acid (C9H8O4, aspirin) is crushed and dissolved
in water. The sample is titrated with 0.160 M sodium hydroxide. The titration required an
average volume of 12.50 ml of sodium hydroxide to reach the endpoint.
The purity of the sample
(a) 60%
(b) 95%
(c) 86%
7. To obtain a precipitate that is useful for gravimetric analysis, the analyst tries to obtain
conditions to encourage crystal growth, as opposed to the formation of a colloid.
Which of the following statements aids in the formation of a crystalline precipitate?
(a) The solution is kept cool to slow down the movement of ions to allow crystals to
form.
(b) The solutions are made as dilute as practical to allow crystals to form slowly.
(c) The solutions are mixed rapidly to allow the appropriate ions to make contact
without interference of other materials.
8. A sample containing 0.2784 grams of sodium chloride (NaCl) and 0.5486 grams of
magnesium chloride (MgCl2). The chloride in the sample was precipitated by the addition
of 47.8 mL of a silver nitrate solution. What is the concentration of the silver nitrate
solution?
(a) 0.341M AgNO3
(b) 0.220 M AgNO3
(c) 0.00341 M AgNO3
9. A sample of ore containing manganese weighed 1.2354 grams. This sample was heated
in a solution of nitric acid, which dissolved the manganese producing manganese(III)
nitrate [Mn(NO3)3]. The acidity of the solution was adjusted, and hydrogen sulfide gas
was bubbled into the solution producing 0.2876 grams of manganese(III) sulfide [Mn2S3].
What is the percentage of manganese in the original sample?
(a) 12.41% Mn
(b) 6.19% Mn
(c) 3.09% Mn
10. A sample containing Al2O3 and Na2Cr2O7•2H2O was brought into the laboratory for
analysis. The analyst heated a 2.2846 gram sample of this material to 100 °C for one
hour, at which time only Al2O3 and Na2Cr2O7 remained. This mixture was found to
weigh 2.2312 grams. Calculate the percent chromium in the sample.
(a) 6.746% Cr
(b) 13.50% Cr
(c) 16.97 % Cr
11. In the combustion of phthalic acid [C6H4(COOH)2], how many moles of water are
produced per mole of phthalic acid burned?
(a) 6 mol H2O
(b) 3 mol H2O
(c) 4 mol H2O
12. In gravimetric analysis, the ideal product should be
(a) very pure, soluble, and should possess a known composition.
(b) very pure, insoluble, easily filterable, and should possess an unknown
composition.
(c) very pure, insoluble, easily filterable, and should possess a known
composition.
13. In gravimetric analysis,
(a) the mass of a solution is used to calculate the quantity of the original analyte.
(b) the mass of a product is used to calculate the quantity of the original analyte.
(c) the mass of a product is used to calculate the quantity of the impurity.
14. A 2.000-g sample of a solid mixture containing only PbCl2 (FM 278.1), CuCl2 (FM
134.45), and KCl (FM 74.55) was dissolved in water to give 100.0 mL of solution. First
50.00 mL of the unknown solution was treated with sodium piperidine dithiocarbamate to
precipitate 0.726 8 g of lead piperidine dithiocarbamate:
Then 25.00 mL of the unknown solution was treated with iodic acid to precipitate
0.8388 g of Pb(IO3)2 and Cu(IO3)2.
the wt % of Cu in the unknown mixture is
a) 13.99 %
b) 56.00 %
c) 3.50 %
15. When the high temperature superconductor yttrium barium copper oxide is heated
under flowing H2, the solid remaining at 1000°C is a mixture of Y2O3, BaO and Cu. The
starting material has the formula YBa2Cu3O7–x, in which the oxygen stoichiometry varies
between 7 and 6.5 (x = 0 to 0.5).
Starting with 28.19 mg of YBa2Cu3O7–x, 25.85 mg of solid product remained at
1000°C.
The value of x is
a) 0.4
b) 0.05
c) 0.2
(Hint: express the mmol of oxygen lost in terms of x and
equate that to the mass lost)
ANSWER LIST – List your answers here
No.
Answer
1
c
2
b
3
b
4
b
5
a
6
c
7
b
8
a
9
a
10
a
11
b
12
c
13
b
14
a
15
b