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- mémoire
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Chemistry 60/68 Mock Exam #2 ANSWERS pp. 1
Los Angeles City College Chemistry 60/68 Mock Exam #2ANSWER KEYProfessor Torres Multiple Choice 1. …C10H12O4S(s) + …O2(g)→ …CO2(g) + …SO2(g) + …H2O(g) When the equation above is balanced and all coefficients are reduced to their lowest whole-number terms, the coefficient for O2(g) is A.6 B.7 C.12 D.14 2. Whichof the following does NOT dissolve in water? A.KCl B.(NH4)2S C.Ba(NO3)2D.AgCl 3. Whensolutions of barium nitrate and potassium sulfate are mixed, a white precipitate forms.What is the formula of the precipitate? A.BaSO4B.Ba2SO4C.KNO3D.K(NO3)24. Chemicalsubstances written to the left of an arrow in a chemical equation are called A.coefficients B.products C.reactants D.subscripts 5. Anyreaction in which a compound breaks down to form two or more simpler substances is called A.Synthesis B.Decomposition C.Single Replacement D.Metathesis
Chemistry 60/68 Mock Exam #2 ANSWERS pp. 2
-6. Whichof the following produces Brions when dissolved in water? A.Br2B.PCl3C.NaBr D.Hg2Br2For Questions 7 – 14, consider the diagram shown below:
B C A D E eaction Cooinate 7. Whichletter represents the enthalpy change of theforwarddirection?D 8. Whichletter represents the activation energy of thereversereaction?C 9. Whichletter represents the transition state of theforwardreaction?B 10. Whichletter represents the activation energy of theforwardreaction?A 11. Whichletter represents the enthalpy change of thereversereaction?D 12. Whichletter represents the transition state of thereversereaction?B 13. MARK“A” for TRUE or “B” for FALSE: Theforwardreaction is an example of an endothermic reaction.A 14. MARK“A” for TRUE or “B” for FALSE: Thereversereaction is an example of an endothermic reaction.B15. Whichof the following represents the synthesis of HF(g) from hydrogen gas and fluorine gas, an exothermic process? A.H2 +F2 +energy→ 2HF B.2HF→ H2 +F2 +energy C.H2F +2→ 2HF+ energy D.2H + 2F→+ energy 2HF
Chemistry 60/68 Mock Exam #2 ANSWERS pp. 3
16. Themolar mass of C3H8O is A.29.02 amu B.53.03 amu C.66.08 amu D.60.09 amu 17. 6.0g of nitrogen (N2) contains A.6.0 atoms of N 23 B.1.26 x 10atoms of N C.0.21 mole of N 23 D.1.26 x 10molecules of N218. Whatis the percent composition of zinc chloride, ZnCl2? A.52.02% Zn, 47.97% Cl B.47.97% Zn, 52.02% Cl C.64.86% Zn, 35.17% Cl D.35.17% Zn, 64.86% Cl 19. Considerthe following reaction: 2C2H4+ 6O2→ 4CO2+ 4H2manyO. How gramsof O2are required for the complete reaction of 45 g of C2H4? 2 A.1.5 x 10 B.51.4 C.112.5 D.288 20. Ina REDOX reaction, A.reduction is the loss of electrons, and oxidation is the gain of electrons. B.no electrons are transferred. C.oxidation is the loss of oxygen, and reduction is the gain of electrons. D.oxidation is the loss of electrons, and reduction is the gain of electrons. Free Response 21. Themolecular formula of aspartame, the artificial sweetener marketed as ® NutraSweet ,is C14H18N2O5. A.Determine the number of moles of aspartame in an 882 mg sample. 27 B.Determine the mass (in g) of aspartame in 9.34 x 10molecules.
Chemistry 60/68 Mock Exam #2 ANSWERS pp. 4
C.How many oxygen atoms are present in 4.33 mg of aspartame? A.mol aspartameg xmol = 0.00300mol aspartame = 882 mgx 1 1000 mg294 g 27 B.mass aspartame = 9.34 x 101 molmolecules xgx 294 23 6.02x 10molec. 1mol 6 =4.56 x 10g aspartame23 C.oxygen atoms =molex 1 mol x 5 mol O x 6.022 x 101 g4.33 mg x 1000mg 294g 1mol cpd1 mol O19 = 4.43 x 10atoms O 22. Considerthe unbalanced chemical equation shown below: 2Al(l) + Cr2O3(s) + heat→ Al2O3(s) +2Cr(l) A.What mass of aluminum is needed to reduce 10.0 kg of chromium(III)oxide to produce chromium metal? B.How many molecules of chromium metal are produced? A.g Al = 10.0 kg Cr2O3x 1 mol Crx 1000 g2O3x 2 mol Alx 27g 1kg 152g 1mol 1mol Al 3 =g Al3.55 x 10B.molec. Cr = 10.0 kg Cr2O3 xx 11000 gmol Cr2O3 x2 mol Cr 1kg Cr2O3 152 g Cr2O3mol Cr 12O323 x6.022 x 10molecules Cr 1mol Cr 25 =7.92 x 10molecules Cr 23. FIGUREIT OUT!One of the common organic chemistry reactions worth mentioning involves the synthesis of aspirin (C9H8O4) from salicylic acid (C7H6O3) and acetic anhydride (C4H6O3balanced reaction proceeds as). The follows: H H O O C OHO CHO C3 HC CHC CC + C C + C H CO CH 3 3 HC C OHHC CO HC OH 3 OH C CC C H H O O Suppose a student obtained an 84.5% yield of aspirin during her experiment. Determine the minimum volume of acetic anhydride (note: density acetic
Chemistry 60/68 Mock Exam #2 ANSWERS pp. 5
anhydride = 1.080 g/mL) that this student would have to use in order to produce 152 kg of aspirin. [***NOTE:asp = aspirin; AA = acetic anhydride] 84.5% = 100% x (152 kg/X) X = 180. kg asp mL AA = 180. kg aspx 1000g aspx 1mol aspx 1mol AAx 102g AAx LAA 1kg asp180 g asp1 mol asp1 mol AA1.080 g 4 =9.44 x 10mL acetic anhydride (AA)24. Considerthe various descriptions below.Predict the product(s) where appropriate,balance the overall reactions, and include net ionic equations if applicable.Write NR if no reaction occurs. A.Solid magnesium turnings are mixed with chlorine gas. Mg(s) + Cl2(g)→ MgCl2(s) B.Solid manganese is reacted with hydrobromic acid. + +2 Mn(s) + 2H(aq)→(aq) + H Mn2(g) C.Iron metal is added to a solution of aluminum phosphate. cannot replace Al!NR! Fe D.Aqueous ammonium sulfide is mixed with aqueous lead(II) nitrate. -2 +2 S (aq) + Pb(aq)→ PbS(s) E.Aqueous radium chloride is mixed with aqueous copper(I) sulfate. +2 -+ -2 (aq) + SORa (aq)+ Cl (aq) + Cu4(aq)→RaSO4(s) + CuCl(s) F.Aqueous scandium(III) chloride is mixed with aqueous nickel(II) nitrate. NR! Bothproducts are soluble, and all spectator ions cross out! G.Bromine gas is reacted with aqueous sodium iodide. - -Br2(g) + 2I(aq)→+ I 2Br (aq)2(g) H.TNT, a powerful organic explosive with the formula C7H5(NO2)3, iscombusted to form molecular nitrogen as well as other expected products. 4C7H5(NO2)3 +21O2→ 6N2 +28 CO210H +2O + heat
Chemistry 60/68 Mock Exam #2 ANSWERS pp. 6
25. Twohydrocarbons have the percent composition 92.25% C and 7.75% H. A.Obtain the empirical formula corresponding to this composition. B.One of the compounds has a molecular weight of 52.03 amu; the other has a molecular weight of 78.05 amu.Obtain the molecular formulas of both compounds. A.You can figure this part out yourself!Convert the percentages (i.e. grams) of carbon and hydrogen to moles, and divide by the smallest number to obtain the empirical formula CH. B.The first compound has a molecular formula of C4H4; the second compound has the molecular formula C6H6. 26. Aspace shuttle uses aluminum metal and ammonium perchlorate to form aluminumoxide, aluminum chloride, nitrogen monoxide, and water in its reusable boosterrockets. Thereaction mixture contains 5.75 g of Al and 7.32 g of NH4ClO4. Al + NH4ClO4→ Al2O3 +AlCl3 +NO + H2O A.Balance the aforementioned chemical equation. B.Determine the theoretical yield (in g) of aluminum chloride, AlCl3. CLEARLYidentify both the LIMITING REAGENT and the EXCESSREAGENT. C.How much (in g) of which reactant is left unreacted? D.If 1.87 g of aluminum chloride is formed, what is the percentage yield of this reaction? A.3Al + 3NH4ClO4→ Al2O3 +AlCl33NO + 6H +2O AlClB. g3= 5.75 g Al x 1 mol Al x 1 mol AlCl3x 133 g AlCl3 27g Al3 mol Al1 mol AlCl3 =9.44 g AlCl3from Al g AlCl3= 7.32 g NH4ClO4x 1 mol x 1 mol AlCl3133 g x 117.5g 3mol NH4ClO4 1 mol AlCl3= 2.76 g AlCl3from NH4ClO4⇐ LIMITING! Therefore,the LIMITING REAGENT is NH4ClO4and the EXCESSREAGENT is Al. reacted Al = 2.76 g AlClC. g3x 1 mol AlCl3x 3 mol Al x 27 g Al 133 g1 mol AlCl31 mol Al =1.68 g reacted Al
Chemistry 60/68 Mock Exam #2 ANSWERS pp. 7
Therefore,g unreacted Al = 5.75 total Al g – 1.68 g Al used =4.07 g unreacted Al D.Percentage Yield = 100% (Actual Yield/Theoretical Yield) =100% (1.87 g/2.76 g) =68.0% yield + 2+2+ 27. Asolution contains one or more of the following ions: Ag, Ca, and/or Cu. Whensodium chloride is added to the solution, no precipitate forms.When sodiumsulfate is added to the solution, a white precipitate occurs.The precipitate isfiltered off, and sodium carbonate is added to the remaining solution, producing aprecipitate. Determinewhich ion(s) is/are present in the original solution? Brieflyexplain your reasoning, and write net ionic equations for the formation of eachof the precipitates observed. When sodium chloride is added to the solution, no precipitate forms: This + statementconfirms that NO Agis present since AgCl(s) would precipitate accordingto our solubility rules! When sodium sulfate is added to the solution, a white precipitate occurs: This +2 statementimplies that Cais present due to formation of the calcium sulfate precipitate.The net ionic equation is as follows: +2 -2 Ca (aq)+ SO4(aq)→ CaSO4(s) …and sodium carbonate is added to the remaining solution, producing a +2 precipitate: Thisstatement implies that Cuis present due to formation of thecopper(II) carbonate precipitate.The net ionic equation is as follows: +2 -2 Cu (aq)+ CO3(aq)→ CuCO3(s) 2+ 2+ Therefore,the unknown contains both Caion and the Cuion.
