Chemistry 111
Practice EXAM 1B
NAME: ____ANSWER KEY____________
SECTION #: ______________
1. ___C__How many significant figures are in 0.01020?
(a) 2
(c) 4
(b) 3
(d) 5
2. ___B__An example of a chemical property is:
(a) Color
(c) Melting point
(b) Flammability
(d) Density
3. ___B__Which of the following forms of radiation would be attracted to a
positively charged plate?
(a) α particle
(c) γ particle
(b) β particle
(d) none of these choices
4. ___B__How many neutrons are in the following isotope: 131I ?
(a) 53
(c) 127
(b) 78
(d) 131
5. ___A__Which of the following is the correct formula for potassium nitrate?
(a) KNO3
(c) KNO2
(b) K3N
(d) KCN
6. ___D__The sodium atom typically loses 1 electron to become:
(a) A larger sodium anion
(c) A larger sodium cation
(b) A smaller sodium anion
(d) A smaller sodium cation
7. ___C__Sodium azide, a shock-sensitive solid, is commonly used in automobile
air bags because of its ability to rapidly release N2 gas according to the following
unbalanced equation: NaN3(s) → Na(s) + N2(g)
How much N2 will be produced if 84.5 g of 93.7% pure NaN3 reacts completely:
(a) 24.3 g
(c) 51.2 g
(b) 34.1 g
(d) 79.2 g
8. ___B__How many grams of carbon dioxide are generated in the combustion of
43.0 g of hexane (C6H14(l)) in air?
(a) 63.0 g
(c) 216 g
(b) 132 g
(d) 301 g
9. ___B__If 3.62 g of hydrogen gas reacts with 7.01 g of nitrogen gas to form
ammonia gas, what is the limiting reagent?
(a) Hydrogen gas
(c) Ammonia gas
(b) Nitrogen gas
(d) Water vapor
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10. ___C__What is the percent yield of a reaction in which 19.78 g of NiCO3
produces 11.6 g of NiO?
(a) 58.6%
(c) 93.2%
(b) 78.3%
(d) 98.9%
11. ___D__Which of the following would not dissolve in water?
(a) Li3PO4
(c) CuCl2
(d) SrSO4
(b) Pb(NO3)
12. ___A__What is the percentage by mass of copper in copper (II) sulfate?
(a) 39.8%
(c) 57.0%
(b) 43.6%
(d) 66.4%
13. ___B__Hydrogen will be displaced from cold water by all of the following
except:
(a) Li
(c) Na
(b) Sn
(d) K
14. ___B__Choose the most appropriate definition for equivalence point from below:
(a) The point when you have used all of the solution in your buret and must
now refill with standard solution.
(b) The point at which you have stoichiometrically equivalent quantities of
reactants.
(c) The point at which you are ready to add the indicator to the solution you
are titrating.
(d) The point at which the eriochrome black T exhibited a bright yellow tinge
in the analysis of bottled water samples.
15. ___A___What was the gas given off during the reaction of your seltzer tablet?
(a) Carbon dioxide
(c) Hydrogen
(b) Oxygen
(d) Sulfur trioxide
Part 2(40 pts total). Partial credit may be given where appropriate. Show all work.
16. Caffeine, a stimulant commonly found in coffee, tea, and chocolate, contains (by
mass) 49.48% C, 5.15% H, and 28.87% N, with the balance being oxygen.
(a) Find the empirical formula of caffeine. (6 pts)
Assume a 100 g sample:
100 g (.4948) = 49.48 g C; mols C = (49.48 g)/(12 g/mol) = 4.123 mol
100 g (.0515) = 5.15 g H; mols H = (5.15 g)/(1 g/mol) = 5.15 mol
100 g (.2887) = 28.87 g N; mols N = (28.87 g)/(14 g/mol) = 2.06 mol
100 g (.1650) = 16.50 g O; mols O = (49.48 g)/(16 g/mol) = 1.03 mol
C(4.123/1.03)H(5.15/1.03)N(2.06/1.03)O(1.03/1.03) = C4H5N2O Empirical formula
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(b) If there are 4 N atoms in each caffeine molecule, what is the molar mass of
caffeine? (3 pts)
If each molecule really has 4 N atoms, then to get from empirical formula
to molecular formula, we multiply all subscripts by 2. This gives:
C8H10N4O2
The molar mass is the mass of 1 mole. Therefore,
MW = 8(12 g/mol) + 10(1 g/mol) + 4(14 g/mol) + 2(16 g/mol)
MW = 194 g/mol
(c) If your 100. g chocolate bar is 0.24% caffeine by mass, how many
caffeine molecules are present in this bar? (4 pts)
First, find mass of caffeine present – the % was different for most exams.
Mass caffeine = (mass bar) x (% as decimal) = (100 g)*(0.0024) = 0.24 g
Moles caffeine = (mass caffeine)/(MW caffeine – from Part C above)
= (0.24 g)/(194 g/mol) = 0.00124 mol
Molecules of caffeine = (mols caffeine)*(NA)
= (1.24 x 10-3 mols)*(6.022 x 1023 molecules/ mol)
= 7.4 x 1020 molecules
17. For the following reaction:
Ba(OH)2(aq) + HClO2(aq) → Ba(ClO2)2(aq) + H2O(l)
(a) Name each reactant and product from above. (2 pts)
Reactants:
Products:
Barium hydroxide + chlorous acid Barium chlorite and water (dihydrogen monoxide)
(b) Write a balanced complete ionic equation. (2 pts)
Ba2+(aq) + 2 OH-(aq) + 2 H+(aq) + 2 ClO2-(aq) → Ba2+(aq) + 2 ClO2-(aq) + 2 H2O(l)
(c) Write a balanced net ionic equation. (2 pts)
2 OH-(aq) + 2 H+(aq) → 2 H2O(l)
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(d) If 7.15 g of Ba(OH)2 is added to sufficient water to yield 125 mL of
solution, what is the minimum amount, in L, of 0.21 M HClO2 that must
be added to neutralize the Ba(OH)2? (4 pts)
Mols Ba(OH)2 = (7.15 g)/(171.33 g/mol) = 0.0417 mols
Mols HClO2 required = (mols Ba(OH)2)*[(2 mols HClO2)/(1 mol Ba(OH)2)]
= 0.835 mols
M = mols/V (in L);
V = mols/M = 0.835 mols/(0.21 mols/L)
V = 0.397 L
V = 0.40 L (2 sig figs only since that’s what HClO2 M is given to(0.21 M))
(e) If phenolphthalein is added to the Ba(OH)2 solution prepared in Part (d)
above and this solution is then titrated with HClO2, how will you know
when the neutralization is complete (be explicit!)? (2 pts)
The solution will change in color from pink to colorless when the
neutralization is complete. The color starts out pink because the
phenolphthalein indicator is added to the basic solution and it appears pink
in base. Once enough acid is added, the solution will no longer be basic
and the solution will go from pink to colorless since phenolphthalein
appears colorless in slightly acidic solutions.
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