Download KEY + + - UIC Department of Chemistry

Chemistry 112
Exam 1B, Form B
Harwood
KEY
1. The following diagram represents the reaction of A2 (shaded spheres) with B2 (unshaded
spheres). How many moles of product can be made from 1.0 mol of A2 and 1.0 mol of B2 ?
(a) 1.0 mol product
(b) 3.0 mol product
(c) 2.0 mol product
(d) 0.67 mol product
2. A reaction between an acid and a metal produces __________.
(a) salt + water
(b) salt + H2(g)
(c) salt + CO2(g)
(d) salt + CO2(g) + H 2 O
3. A compound was found to contain 54.5% carbon, 9.2% hydrogen, and 36.3% oxygen by
mass. What is the empirical formula of the compound?
(a) C6 HO4
(b) C4 H8 O2
(c) C2 H4 O
(d) C3 H4 O
(e) C3 H4 O3
Questions 4 – 6 refer to the following unbalanced equation:
Fe(s)
+
H2 O(l)
→
Fe2 O4(s)
+
H2(g)
4. Identify the limiting reactant when 5.0 g of Fe react with 5.0 g of water.
(a) Fe
(b) H2 O
5. What mass of Fe2 O4 can be produced from the amounts of Fe and water in question 4?
(a) 48.8 g
(b) 49 g
(c) 7.86 g
(d) 7.9 g
(e) 12 g
6. Given the answer in question 5, what is the actual yield of the reaction if the percentage yield is
82%?
(a) 6.4 g
(b) 60 g
(c) 10 g
(d) 40 g
7. How many atoms of hydrogen are in 48.2 g of CH4 ?
–23
26
(a) 2.00 × 10
(b) 12
(c) 1.16× 10
24
(d) 7.24 × 10
(e) 16
8. Consider the equation shown below. If 3.0 g of O2 produces 3.4 g of H2 O, how many grams
of H2 O would be produced by 6.0 g of O2 ? 2 H2 + O2 → 2 H2 O
(a) 5.3 g
(b) 1.7 g
(c) 6.4 g
(d) 3.4 g
(e) 6.8 g
9. The reaction of AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) is best classified as
(a)
(b)
(c)
(d)
an acid-base neutralization reaction
a dissolution reaction
a precipitation reaction
none of the above
1
Chemistry 112
Exam 1B, Form B
Harwood
KEY
10.Which net ionic equation would be consistent with the following reaction?
Li2 CO3(aq) + CaCl2(aq) → ?
+
–
(a) Li (aq) + Cl (aq) → LiCl(s)
2+
2–
(b) Ca (aq) + CO3 (aq) → CaCO3(aq)
2+
2–
(c) Ca (aq) + CO3 (aq) → CaCO3(s)
+
–
(d) Li (aq) + Cl (aq) → LiCl(aq)
+
2–
(e) Li (aq) + CO3 (aq) → Li2 CO3(aq)
11.What are the spectator ions in the reaction between KOH(aq) and HNO3(aq)?
+
+
+
–
+
–
+
–
(a) K and H
(b) H and OH
(c) K and NO3
(d) H and NO3
12.Ammonia (NH3 ) burns in oxygen gas to form nitrogen gas and water.
a. Write a balanced equation for this reaction.
(4 points)
4 NH3 + 3 O 2 → 2 N2 + 6 H2O
b. How many moles of oxygen are needed to react completely with 1.50 g of ammonia?
(6 points)
mole NH 3 = (1.50 g)(1 mol / 17.0304 g) = 0.0880778 mol NH3
mol O 2 = (0.0880778 mol NH3)(3 mol O2 / 4 mol NH3) = 0.0661 mol O 2
c. Starting with the 1.50 g of ammonia in part b above and an excess of O2 , you obtain 1.80 g
of product which you believe to be N2 . Is that possible? Explain your answer. (7 points)
mass N2 possible = (0.0880778 mol NH3)(2 mol N2/4 mol NH3)(28.0134 g/1 mol N2)
= 1.23 g N 2 (theoretical yield)
Not possible to form 1.80 g N2. Can't make more N2 than the theoretical yield.
13.Separate samples of a solution of an unknown salt are treated with dilute solutions of HBr,
H2 SO4 , and NaOH. A precipitate forms only with H2 SO4 . Which of the following cations
+
2+
2+
could the solution contain: K ; Pb ; Ba ? Briefly explain.
(6 points)
2+
Ba . All potassium salts are soluble. BaSO4 and PbSO4 are both insoluble. PbBr2
is also insoluble. The problem states that a precipitate forms ONLY when H2SO 4 is
2+
added. If the cation were Pb then a ppt would form when either H2SO 4
or HBr was added.
BONUS QUESTION
A compound of formula XCl3 reacts with aqueous AgNO3 to yield a precipitate of solid AgCl
according to the following equation:
XCl3(aq) + 3 AgNO3(aq) → X(NO3 )3(aq) + 3 AgCl(s)
When a solution containing 0.634 g of XCl3 was allowed to react with an excess of aqueous
AgNO3 , 1.68 g of solid AgCl was formed. What is the identity of the atom X?
(10 points)
moles AgCl = (1.68 g)(1 mol AgCl/143.3212 g) = 0.011722 mol AgCl
moles XCl 3 = (0.011722 mol AgCl)(1 XCl3 / 3 AgCl) = 0.0039073 mol XCl3
molar mass XCl3 = 0.634 g / 0.0039073 mol = 162.26 g/mol
molar mass X = 162.26 – 3(35.453) = 55.90 g/mol
X = Fe
2