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Chem 1050 Core Chemical Concepts I

Chemical calculations: especially basic stoichiometry, limiting reagents, gravimetricanalysis, empirical and molecular formulas

Chemical reactions: writing balanced equations and predicting products

Important: These problems (which are organized by topic) are intended to be a guideto bring you up to the required competency for this course. You should dosome problems in all areas indicated and concentrate on topics which youhave forgotten or have not already encountered.

Review problems:

Calculations involving moles, Avogadro's number, density, mass percent.

I Calculate the mass of 5.58 x 1022 formula units of NaCl.

II. The density of ice is 0.917 g mL–1 at 0ºC and 101.3 kPa of pressure. Calculatethe volume occupied by 1.000 kg of ice under these conditions.

III. An aqueous solution is 15.0% NaCl by mass and has a density of 1.15 g mL–1.

(a) Calculate the mass in grams of 300.0 mL of this solution.(b) Calculate the mass of NaCl present in 300.0 mL of this solution.

Calculations involving solutions, solution preparation and dilution.

1 . Calculate the molar concentration of the solute in the following solutions:

(a) 12.42 g of HCl dissolved in enough H2O to make 250.0 mL of solution

(b) 3.618 g of Cd(NO3)2 dissolved in enough H2O to make 2.000 L of solution.

2. Calculate the mass of Kl required to make 50.00 mL of 0.08000 mol L–1 Kl(aq).

3. Calculate the volume (in mL) of 0.2002 mol L–1 KMnO4(aq) which contains 7.140 g of KMnO4.

4. How many milliliters of 6.00 mol L–1 HNO3 (aq) are required to make 500.0 mL of0.255 mol L–1 HNO3(aq)?

5. A student wishes to dilute 3.0 mL of 2 mol L–1 NaOH(aq) to a concentration of 0.1mol L–1. Calculate the final volume of solution required.

6. A student adds 100.0 mL of 0.800 mol L–1 NaCl(aq) to 400.0 mL of 0.800 mol L–1 KBr(aq). Calculate the molar concentrations of NaCl and KBr in thecombined solution.

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Percent composition, empirical and molecular formulas.

7. Calculate the % by mass of oxygen (to 4 significant figures) in the followingcompounds:

(a) K4P2O7 (b) Be(NO3)2A4H2O

8. Carboranes are a class of compounds composed of carbon, boron and hydrogen.One carborane is found to contain 32.77% C and 59.00% B by mass. Determineits empirical formula.

9. Ethyl butyrate, a compound containing carbon, hydrogen and oxygen is responsiblefor the characteristic odor of pineapples. Determine the empirical formula of ethylbutyrate if combustion of 2.78 g of ethyl butyrate produced 6.32 g CO2 and 2.58 gH2O. Find its molecular formula if its molar mass has been determined to be116.2 g molG1.

10. When a 6.20 g sample of a compound containing only carbon, hydrogen and sulfurreacted with an excess of Cl2, 16.7 g of HCl and 26.4 g of CCl4 were obtained.Determine the empirical formula of the compound.

11. A 6.00 g sample of cobalt metal is completely consumed in a stream of oxygenproducing 8.17 g of compound. Determine the empirical formula of the cobalt-oxygen compound formed.

Balancing equations.

12. Balance the following equations by inspection:

(a) CuO(s) + NH3(aq) Cu(s) + H2O(l) + N2(g)→

(b) C4H6O2(s) + O2(g) CO2(g) + H2O(l) →

(c) CaS(aq) + Bi(NO3)3(aq) Ca(NO3)2(aq) + Bi2S3(s)→

(d) La2O3(s) + H2O(l) La(OH)3(s)→

(e) Ca(OH)2(aq) + HBr(aq) CaBr2(aq) + H2O(l) →

(f) Zn(BrO3)2(s) ZnO(s) + Br2(l) + O2(g)→

(g) C3H5NO(s) + O2(g) CO2(g) + H2O(g)+ NO2(g)→

(h) P4O6(s) + H2O(l) H3PO3(aq)→

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(i) Ag+(aq) + Al(s) Ag(s) + Al3+ (aq)→

Basic stoichiometry problems.

13. Given the following equation for the combustion of ammonia with oxygen at hightemperatures: 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g)→

(a) If 75.4 g of ammonia are consumed, calculate the minimum mass of oxygenrequired and the maximum masses of NO and H2 that can be formed.

(b) If 338 g of H2O are formed, calculate the maximum mass of NO formed and theminimum masses of ammonia and oxygen which must be consumed.

14. Given the following equation representing the reaction of copper(II)oxide withammonia:

3 CuO(s) + 2NH3(g) 3 Cu(s) + N2(g)+ 3 H2O(g)→

Determine the percentage yield of copper metal if 48.34 g of copper are collectedfrom the reaction of 9.44 g of ammonia with an excess of copper(II)oxide.

15. Given: 2 AgNO3(aq) + Na2S(aq) Ag2S(s) + 2NaNO3(aq)→

Calculate the mass of Ag2S(s) formed when 35.0 mL of 0.200 mol L–1 AgNO3(aq)are treated with an excess of sodium sulfide solution.

Limiting reagent problems.

16. Potassium nitrate, KNO3, sometimes called saltpeter, is an important fertiliser.One process for its manufacture is the following reaction, carried out at elevatedtemperatures:

3 KCl(s) + 4 HNO3(l) Cl2(g) + NOCl(g) + 2 H2O(g) + 3 KNO3(s)→

(a) Determine the limiting reagent when 101.0 g of both reactants are mixed.Justify your answer.

(b) Assuming the limiting reagent is totally consumed, calculate the mass of theexcess reagent which remains unreacted.

(c) Calculate the maximum mass of KNO3 that can be formed.

17. Given: 3 Cu(s) + 8 HNO3(l) 3 Cu(NO3)2(aq) + 8 H2O(l) + 2 NO(g)→

(a) If 8.41 g of Cu(s) are added to 150.0 mL of 2.00 mol L–1 HNO3(aq),calculate the maximum amount in grams of NO(g) that can be formed.

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(b) If 2.04 g of NO(g) are actually collected, calculate the % yield of NO(g).

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18. Manganese can be isolated from MnO2 by the thermite reaction, in which the oxidereacts with aluminum metal forming Mn and A12O3 and liberating a large amount ofheat. A mixture of 227g of MnO2 and 73.4g of Al is allowed to react to completion.Calculate the actual yield of Mn if the % yield of the reaction is 75.0 %.

3 MnO2(s) + 4 Al(s) 3 Mn(s) + 2 Al2O3(s)→

Gravimetric analysis.

19. A 4.270 g sample of an unknown metal bromide XBr3 was dissolved in water andtreated with excess AgNO3. If 8.140 g of AgBr were recovered,

(a) Calculate the number of moles of AgBr formed.

(b) Calculate the number of moles of XBr3 reacted.

(c) Calculate the molar mass of XBr3.

(d) Calculate the molar mass of element X.

(e) Identify element X.

20. A 3.500 g sample of a compound of cobalt and iodine was dissolved in water andtreated with excess AgNO3 solution producing 5.255 g of Agl precipitate.Determine the empirical formula of the compound.

21. A 2.714 g sample of the hydrated salt, CuSO4AXH2O was dissolved in water andtreated with excess Ba(NO3)2 solution producing 2.537 g of BaSO4 precipitate.Determine the value of X in the formula CuSO4AXH2O.

Acid/base neutralization stoichiometry problems.

22. A 20.17 mL aliquot of 0.3036 mol L–1 NaOH(aq) completely neutralizes 25.00 mLof H2SO4(aq). Calculate the molar concentration of the H2SO4(aq). Hint: Writea balanced equation for the reaction.

23. Vitamin C, ascorbic acid, H2C6H6O6, (a weak diprotic acid), can be taken orally intablet form where it is the main ingredient. A vitamin C tablet with a mass of0.4902 g is dissolved in sufficient deionized H2O and neutralized by 20.14 mL of0.2742 mol L–1 NaOH(aq). The neutralization reaction can be represented asfollows:

H2C6H6O6(aq) + 2 NaOH(aq) Na2C6H6O6(aq) + 2 H2O(l) →

(a) Calculate the mass of pure ascorbic acid which reacted.

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(b) Calculate the % by mass of ascorbic acid (i.e. % purity) in the vitamin Ctablet.

24. The active ingredient in common aspirin is acetylsalicylic acid (ASA), a weakmonoprotic acid with the formula HC9H7O4. The aspirin contains ASA and annon!medicinal binder. A 0.8621g tablet is dissolved in sufficient deionized H2Oand then neutralized by 32.05 mL of 0.07070 mol L–1 Ba(OH)2(aq) according to thefollowing equation:

2 HC9H7O4 (aq) + Ba(OH)2(aq) Ba(C9H7O4)2(aq) + 2 H2O(l)→

Calculate the mass percent of acetylsalicylic acid in the tablet.

25. Calculate the volume in milliliters of 0.3450 mol L–1 KOH(aq) required for thecomplete neutralization of 38.55 mL of 0.1275 mol L–1 H3PO4(aq). Hint: Write abalanced equation for the reaction.

Strong electrolytes.

26. A 2.56 g sample of Na2SO4A10H2O is dissolved in enough water to make 250.0mL of solution. Calculate the molar concentrations of Na+(aq) and SO4

2– (aq) in thesolution.

27. A solution is made by dissolving 0.800 g of NaCl and 1.60 g of CaCl2 in enoughwater to make 100.0 mL of solution. Calculate the molar concentration of chlorideion (Cl–) in the solution.

28. A 10.00 mL volume of 0.160 M Al2(SO4)3(aq) is mixed with a 15.00 mL volume of0.320 M Na2SO4(aq). Determine the concentration of all three ions in the finalsolution.

Miscellaneous problems.

29. A 10.00 mL aliquot of vinegar (density = 1.02 g mLG1) is diluted to 100.0 mL withdeionized water in a volumetric flask. A 25.00 mL sample of the diluted vinegarrequires 9.10 mL of 0.2218 M NaOH(aq) to reach a pink phenolphthalein endpoint. Calculate the mass % of acetic acid, HC2H3O2, in the undiluted vinegar.

30. An iron carbonyl, Fe3(CO)X, is 33.26% Fe by mass. Determine the value of x for inthe formula.

31. An ionic compound, KBrOx, where x is unknown, is analysed and found to contain52.92% bromine by mass. Determine the value of x and name the compound.

32. A compound of metal M and oxygen has the empirical formula M3O4 and is 72.36%M by mass. Calculate the molar mass of metal M and hence determine its identity.

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33. If 34.2 g of Ca(NO3)2A4H2O is dissolved in water and reacted with excessNa3PO4(aq), what mass of Ca3(PO4)2 will be produced given the following netionic equation?

3 Ca2+ (aq) + 2 PO43-(aq) Ca3(PO4)2(s)→

34. Potassium dichromate can be prepared by reacting chromium(III)chloridehexahydrate with hydrogen peroxide under basic conditions. When 8.00 g ofchromium(III) chloride hexahydrate reacts with 9.022 g of KOH and 10.0 mL of6.00 mol L–1 H2O2(aq), 2.77 g of potassium dichromate is recovered. Calculatethe percentage yield of potassium dichromate. The net ionic equation for thereaction is:

2 Cr+3 (aq) + 3 H2O2(aq) + 8 OH–(aq) Cr2O72– (aq) + 7 H2O(l) →

35. An 8.000 g sample of a compound containing carbon, hydrogen, and nitrogenwas burned in excess oxygen gas producing 15.83 g of carbon dioxide and 5.835g of water. Determine the empirical formula of the compound.

36. A crucible containing some limestone, CaCO3, has a total mass of 27.133 g. Thecrucible is heated to decompose all the limestone according to the followingequation:

CaCO3(s) CaO(s) + CO2(g)heat →

After the decomposition was complete, the crucible was cooled to roomtemperature and reweighed giving a mass of the crucible and its contents of26.847 g. Calculate the mass of the crucible.

37. An element X forms an iodide XI3 and a chloride XCI3. The iodide is quantitativelyconverted to the chloride when it is heated in a stream of chlorine:

2 XI3 + 3 Cl2 6 2 XCl3 + 3 I2

If 0.5000 g of XI3 is treated, 0.2360 g of XCl3 is obtained.

(a) Calculate the molar mass of element X

(b) Identify element X.

38. A mixture containing KClO3, KHCO3, K2CO3, and KCl was heated strongly,producing CO2, O2, and H2O gases according to the following equations:

2 KClO3(s) 2 KCl(s) + 3 O2(g)→

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2 KHCO3(s) K2O(s) + 2 CO2(g) + H2O(g)→

K2CO3(s) K2O(s) + CO2(g)→

The KCl is very stable and does not decompose under the conditions of theexperiment. If 100.0 g of the mixture produces 1.80 g of H2O, 13.20 g of CO2, and4.00 g of O2, calculate the mass percent of KCl in the mixture.

39. A 25.00 mL aliquot of a mixture containing 0.03068 M HCl(aq) and an unknownconcentration of H2SO4(aq) requires 27.54 mL of 0.1107 M NaOH(aq) tocompletely neutralize the acid mixture. Calculate the concentration of the H2SO4 inthe mixture in mol LG1.

40. A 1.9791 g sample of MnCl2AXH2O was heated to drive off the water of hydration,leaving 1.2584 g of solid residue. Determine the value of X in the formula of thehydrate.

41. Calculate the [Na+] in a solution made by mixing 50.0 mL of 0.200 mol L–1

Na2SO4(aq) with 200.0 mL of 0.100 mol L–1 Na3PO4(aq).

42. A 25.00 mL sample of HCl(aq) was added to a 0.1000 g sample of CaCO3. All theCaCO3 reacted, leaving some unreacted HCl(aq).

CaCO3(s) + 2 HCl(aq) CaCl2(aq) + H2O + CO2(g)→

The unreacted HCl(aq) required 43.82 mL of 0.01185 M Ba(OH)2(aq) to completethe following reaction.

2 HCl(aq) + Ba(OH)2(aq) BaCl2(aq) + 2 H2O(l) →

Calculate the molar concentration of the original HCl(aq).

43. The reaction below can be used as a laboratory method of preparing smallquantities of Cl2(g). When a 62.6 g sample that is 98.5% K2Cr2O7 by mass isallowed to react with 325 mL of HCl(aq) with a density of 1.15 g mLG1 and 30.1%HCl by mass, 38.9 grams of Cl2(g) are produced. Calculate the % yield of chlorinegas.

K2Cr2O7(aq) + 14 HCl(aq) 2 CrCl3(aq) + 3 Cl2(g) + 7 H2O(l) + 2 KCl(aq)→

44. Zn can reduce NO3– to NH3(g) in basic solution as shown below:

NO3–(aq) + 4 Zn(s) + 7 OH–(aq) + 6 H2O(l) 4 [Zn(OH)4]2–(aq) + NH3(g)→

The gaseous NH3 is driven out of the reaction vessel into a second flask where it is

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neutralized with an excess of HCl(aq). Then, the unreacted HCl can be titratedwith NaOH. In this way a quantitative determination of NO3

– can be achieved. A25.00 mL sample of nitrate solution was treated with zinc in basic solution. TheNH3(g) was passed into 50.00 mL of 0.1500 M HCl(aq). The unreacted HClrequired 32.10 mL of 0.1000 M NaOH for its titration. What was the [NO3

–] in theoriginal sample? Hint: Write balanced equations for the reaction of NH3(aq) withHCl(aq) and HCl(aq) with NaOH(aq).

45. (a) Determine the empirical formula of an acid which had the followingelemental composition by mass: Carbon - 48.64%, Hydrogen - 8.16%,Oxygen !43.19%.

(b) The acid in (a) is monobasic (has one ionizable hydrogen) and hence canbe given the formula HX. A 8.558 g sample of the acid was dissolved inwater to produce a total volume of 1000.0 mL. A 25.00 mL aliquot of theacid solution required 28.88 mL of 0.1000 mol L–1 aqueous sodiumhydroxide to react with all the acid. Determine the molecular formula ofthe acid.

46. The molar concentration of nitric acid, HNO3, in a sample of concentrated nitricacid is to be determined.

(a) Initially, a NaOH solution was standardized by titration with a sample ofpotassium hydrogen phthalate, KHC8H4O4, a monoprotic acid (HX) oftenused as a primary standard. A sample of pure KHC8H4O4 weighing 1.518grams was dissolved in water and titrated with NaOH solution. To reach theequivalence point, 26.90 milliliters of base was required. Calculate themolarity of the NaOH solution. (Molecular weight: KHC8H4O4 = 204.2 gmolG1).

HC8H4O4G(aq) + OHG(aq) C8H4O42G(aq) + H2O(l) →

(b) A 10.00 milliliter sample of the concentrated nitric acid was diluted withwater to a total volume of 500.0 milliliters. Then 25.00 milliliters of thediluted acid solution was titrated with the standardized NaOH solutionprepared in part (a). The equivalence point was reached after 28.35milliliters of the base had been added. Calculate the molarity of theconcentrated nitric acid.

47. A hydrated nickel salt is 23.8% Ni, 13.0% S, 58.3% O and 4.9% H by mass. If itsmolar mass is 246.9 g mol–1, determine the formula of the hydrated salt.

48. A 2.500 g sample of a compound of lithium, chlorine and oxygen is heated for anextended period to drive off the oxygen. The residue weighs 1.170 g. It is nowdissolved in water and treated with silver nitrate. All of the chloride precipitates asAgCl, which weighs 3.963 g. Calculate the empirical formula of the lithium-chlorine-oxygen compound.

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49. When M2S3(s) is heated in air, it is converted to MO2(s) and SO2(g). A 4.000 gsample of M2S3(s) is heated in air producing 3.732 g of MO2(s). Calculate themolar mass of M and identify it. Hint: Write a balanced equation for the reaction.

50. An electric furnace produces phosphorus by the following reaction:

2 Ca3(PO4)2(s) + 10 C(s) + 6 SiO2(s) 6 CaSiO3(s) + 10 CO(g) + P4(g)→

An initial reaction mixture containing 750.0 kg calcium phosphate, 125.0 kg carbonand 500.0 kg silicon dioxide. If 95.8 kg of phosphorus is recovered, calculate the% yield of phosphorus.

51. Predict whether or not a precipitate will form when the following aqueous solutionsare mixed using your knowledge of the solubility rules. Write balanced molecular(whole formula) equations for all reactions which yield a precipitate.

(a) AgNO3(aq) + CaCl2(aq)

(b) Na2S(aq) + Mg(C2H3O2)2(aq)

(c) CuSO4(aq) + Fe(NO3)3(aq)

(d) Na2SO4(aq) + BaBr2(aq)

(e) Pb(NO3)2(aq) + HI(aq)

(f) CrCl3(aq) + NaOH(aq)

(g) NH4NO3(aq) + Na3PO4(aq)

(h) Hg2(NO3)2(aq) + NaCl(aq)

(i) K2S(aq) + NiCl2(aq)

(j) Ca(NO3)2(aq) + K2CO3(aq)

(k) Co(C2H3O2)2(aq) + FeCl2(aq)

(l) CaI2(aq) + (NH4)3PO4(aq)

52. Given the following word equations, write molecular (whole-formula form), totalionic (ionic form) and net ionic equations for each:

(a) Aluminum metal is added to a solution of nitric acid yielding hydrogen gasand a solution of aluminum nitrate.

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(b) Aqueous solutions of ammonium phosphate and iron(II) chloride are mixedyielding a precipitate of iron(II) phosphate and aqueous ammoniumchloride.

(c) Magnesium metal is added to a solution of silver nitrate.

(d) The addition of hydrochloric acid to solid sodium carbonate decahydrate.

(e) The complete neutralization of nitric acid by aqueous barium hydroxide.

(f) Chlorine water is added to aqueous potassium bromide.

(g) The complete neutralization of acetic acid by aqueous sodium hydroxide.

(h) The complete neutralization of aqueous ammonia by aqueous nitric acid.

Answers:

I mass NaCl = 5.42 gII 1.09 x 103 mLIII (a) 345 g solution, mass NaCl = 51.8 g1. [HCl] = 1.362 mol LG1, [Cd(NO3)2 = 7.650 x 10-3 mol L–1

2. 0.6640g KI3. 225.7 mL4. 21.3 mL5. 60 mL6. [NaCl] = 0.160 mol L–1, [KBr] = 0.640 mol L–1

7. (a) 33.90% (b) 78.01%8. B2CH3

9. EF = C3H6O MF = C6H12O2

10. C3H8S2

11. Co3O4

12. Coefficients of reactants (R) and products (P) are given below:(a) 3 R1 + 2 R2 6 3 P1 + 3 P2 + P3(b) 2 R1 + 9 R2 6 8 P1 + 6 P2(c) 3 R1 + 2 R2 6 3 P1 + P2(d) R1 + 3 R2 6 2 P1(e) R1 + 2 R2 6 P1 + 2 P2(f) 2 R1 6 2 P1 + 2 P2 + P3(g) 4 R1 + 19 R2 6 12 P1 + 10 P2 + 4 P3(h) R1 + 6 R2 6 4 P1(i) 3R1 + R2 6 3 Pl + P2

13. (a) 177 g O2, 133 g NO, 120 g H2O (b) 375g NO, 213g NH3, 502 g O2

14. 91.5% yield15. 0.868 g Ag2S

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16. (a) HNO3 is LR (b) 11.4g KCl (c) 121.5 g KNO3

17. (a) 2.25 g NO (b) 90.6% yield18. 84.1 g Mn19. (a) n(AgBr) 0.04335 mol (b) n(XBr3) 0.01445 mol

(c) molar mass of XBr3 295.4 g mol–1 (d) molar mass of X = 55.8 g mol–1

(e) X is iron20. Empirical formula is Col221. X = 5, compound is CuSO4A5H2O22. 0.1225 mol L–1 H2SO4(aq)23. (a) 0.4863 g (b) 99.20% purity24. 94.71%25. 42.74 mL26. [Na+] = 0.0656 mol L–1, [SO4

2–] = 0.0328 mol L–1

27. [Cl–] = 0.425 mol L–1

28. [Al3+(aq)] = 0.128 M [Na+(aq)] = 0.384 M [SO42G(aq)] = 0.384 M

29. 4.75% HC2H3O2

30. x = 1231. x = 2, potassium bromite32. M is iron33. 15.0 g Ca3(PO4)2 34. 62.7% yield of K2Cr2O7 35. C5H9N3 36. mass of crucible = 26.483 g37. (a) molar mass of X = 138.9 g mol–1 (b) X is La (atomic # = 57)38. 56.0 % KCl 39. [H2SO4(aq)] = 0.04563 M40. X = 4 41. [Na+] = 0320 mol L–1 42. [HCl] = 0.1215 mol L–1 43. 87.3% yield44. [NO3

–] = 0.1716 mol L–1 45. EF = MF = C3H6O2 46. (a) [NaOH] = 0.2764 mol L–1 (b) [HNO3] = 15.67 mol L–1

47. NiSO3A6 H2O 48. LiClO3 49. Iridium50. 74.3 g P4

51. 2 AgNO3(aq) + CaCl2(aq) 2 AgCl(s) + Ca(NO3)2(aq)→Na2SO4(aq) + BaBr2(aq) BaSO4(s) + 2 NaBr(aq)→Pb(NO3)2(aq) + 2 HI(aq) PbI2(s) + 2 HNO3(aq)→CrCl3(aq) + 3 NaOH(aq) Cr(OH)3(s) + 3 NaCl(aq)→Hg2(NO3)2(aq) + 2 NaCl(aq) Hg2Cl2(aq) + 2 NaNO3(aq)→K2S(aq) + NiCl2(aq) NiS(s) + 2 KCl(aq)→Ca(NO3)2(aq) + K2CO3(aq) CaCO3(s) + 2 KNO3(aq)→3 CaI2(aq) + 2 (NH4)3 PO4(aq) Ca3(PO4)2 (s) + 6 NH4I(aq)→

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52. net ionic equations

(a) 2 Al(s) + 6 H+(aq) 2 Al3+(aq) + 3 H2(g) →(b) 3 Fe2+(aq) + 2 PO4

3–(aq) Fe3(PO4)2(s)→(c) Mg(s) + 2 Ag+(aq) Mg2+(aq) + 2 Ag(s)→(d) 2 H+(aq) + Na2CO3•10 H2O(s) 2 Na+(aq) + CO2(g) + 11 H2O(l) →(e) 2 H+(aq) + OH–(aq) H2O(l) →(f) Cl2(aq) + 2 Br–(aq) 2 Cl–(aq) + Br2(aq) →(g) CH3COOH(aq) + OH–(aq) CH3COO–(aq) + H2O(l) →(h) NH3(aq) + H+(aq) NH4

+(aq) →

Developed by Dr. Chris Flinn