CHEMISTRY 123-07 Midterm #1 – Answer key
October 14, 2010 Statistics:
• Average: 74 p (74%);
• Highest: 97 p (95%); Lowest: 33 p (33%)
• Number of students performing at or above average: 67 (57%)
• Number of students performing below 55%: 17 (15%)
• Number of students at or above 90%: 12 (10%)
PART I: MULTIPLE CHOICE (30 multiple choice questions. Each multiple choice question is worth 2 points) 1. All of the following properties of a substance can aid in its identification except:
a. Density b. Melting point
c. Boiling point d. Temperature 2. Which of the following is a chemical property of iron?
a. Iron melts at 1535 oC.
b. Iron conducts heat.
c. Iron can be bent into shapes.
d. Iron rusts on exposure to water and oxygen.
e. Iron conducts electricity.
3. Which of the following is not part of Dalton's Atomic Theory?
a. An atom of one element can be chemically transformed into a different type of atom.
b. Atoms of different elements combine in whole number quantities.
c. Atoms of a given element have the same mass.
d. Atoms of different elements have different masses.
e. Matter is not created nor destroyed in a chemical reaction; the molecular arrangements are changed.
4. Which of the following represents a pair of allotropes?
a. Chlorine and hydrogen chloride b. Oxygen and oxygen fluoride c. Water and ammonia d. Oxygen and ozone
e. Carbon and carbon tetrachloride
5. The Millikan oil drop experiment was used to determine a. The nuclear character of the atom
b. Whether the atom was radioactive c. The electron distribution in the atom d. The atomic number of an atom e. The charge of an electron
6. Which of the following statements is not true?
a. 51V has an atomic number of 23.
b. 95Mo contains 53 neutrons.
c. 66Zn has the same number of electrons and protons.
d. 56Fe has the same number of neutrons and protons.
e. 12C has the same number of protons, neutrons and electrons.
7. Which statement best describes isotopes?
a. Atoms with the same number of protons and different numbers of neutrons b. Atoms with the same number of neutrons and a different number of protons c. Atoms with the same numbers of protons and neutrons
d. Atoms in the same horizontal period e. Atoms in the same vertical family
8. Which of the following statements concerning isotopes is true?
a. Every element has 3 isotopes.
b. All isotopes of a given element are radioactive.
c. Various isotopes of the same element exhibit very different chemical reactivity.
d. Various isotopes of the same element have the same mass.
e. Various isotopes of the same element will have the same number of protons.
9. Which sample contains the greatest number of atoms (Think conceptually! You do not have to calculate the explicit number of atoms in each case!)?
a. 50 g P b. 50 g Ca c. 50 g Ba 10. Which element can be classified as a transition metal?
a. N b. Ne c. Ce d. Co e. Sr
11. Which of the following compounds has the same percent composition by mass as styrene, C8H8? a. acetylene, C2H2
b. benzene, C6H6 c. cyclobutadiene, C4H4 d. α-ethyl naphthalene, C12H12 e. all of these
12. The empirical formula of a group of compounds is CHCl. Lindane, a powerful insecticide, is a member of this group. The molar mass of lindane is 290.8 g/mol. How many atoms of carbon does a molecule of lindane contain?
a. 2 b. 3 c. 4 d. 6 e. 8
13. Which formula – name combination is incorrect?
a. CS2 carbon disulfide b. Cl2O7 chlorine heptaoxide c. P2O3 diphosphorus trioxide d. SbCl5 antimony pentachloride e. SCl2 sulfur dichloride
14. Find the correct combination of protons and electrons below for the oxide ion.
a. 8 protons and 8 electrons b. 16 protons and 18 electrons c. 8 protons and 10 electrons
d. 8 protons and 6 electrons e. 16 protons and 16 electrons
15. Give the formula for the ionic compound that forms between strontium and nitrogen.
a. SrN b. Sr2N c. Sr3N
d. SrN2 e. Sr3N2
16. Which compound is incorrectly named?
a. Ba(OH)2 barium hydroxide b. Na2SO4 sodium sulfite c. Fe(OH)3 iron(III) hydroxide d. NH4NO3 ammonium nitrate e. K2Cr2O7 potassium dichromate
17. In the reaction given below, if 10 moles of aluminum oxide are consumed, how many moles of oxygen gas are produced?
2 Al2O3 4Al + 3O2
a. 3 b. 4 c. 10
d. 12 e. 15
18. Classify the following reaction.
2 Al(s) + 3 CuSO4(aq) Al2(SO4)3(aq) + 3 Cu (s) a. combination
b. decomposition
c. displacement d. exchange 19. Classify the following reaction.
CaCl2(aq) + H2SO4(aq) CaSO4(s) + 2 HCl (aq) a. combination
b. decomposition
c. displacement d. exchange
20. The Roman numerals in the reaction given represent the coefficients in the balanced chemical equation. What are the values of the coefficients?
I CO + II NO III CO2 + IV N2 I II III IV
a. 2 2 2 3 b. 1 1 1 2 c. 2 1 2 1 d. 1 2 2 1 e. 2 2 2 1
21. Which of the following is a diprotic acid?
a. H3PO4 b. H2O c. Ca(OH)2
d. H2SO4 e. NH3
22. Which compound can be classified as insoluble:
a. Mg(ClO4)2; b. Cu(NO3)2; c. (NH4)2SO4;
d. Fe(OH)2 e. NiSO4;
23. Consider the reaction between 15.0 mL of a 1.00 M aqueous solution of AgNO3 and 10.0 mL of a 1.00 M aqueous solution of K2CO3. When these react, a precipitate is observed. What is present in solution after the reaction is complete (Note: the precipitate is not considered to be in solution)?
a. Ag+, NO3–, K+, CO32–, water b. Ag+, NO3–, K+, water c. K+, CO32–, water
d. NO3–, K+, CO32–, water e. water
24. Which statement about strong acids is true?
a. Strong acids are weak electrolytes.
b. Strong acids are very concentrated.
c. Strong acids are almost entirely converted to ions when dissolved in water.
d. Acetic acid is a strong acid.
e. All of the above are true.
25. Which of the following is a weak electrolyte?
a. HBr b. Mg(OH)2 c. NaCl
d. HCN e. KI
26. What is the correct formula for the ammonium ion?
a. H2O;
b. H3O+; c. OH-
;
d. H-
; e. NH4+;
27. What is the oxidation state (number) of Cl in Cl2O5? a. +2
b. -2
c. -3 d. -5
e. +5
28. What is the oxidation state (number) of P in PO43-
? a. -3
b. +3 c. -2
d. +5 e. –5
29. What is the oxidation state (number) of H in MgH2? a. -1
b. +1 c. 0
d. +2 e. –2
30. What is the oxidation state (number) of F in F2? a. -1
b. +1 c. 0
d. +2 e. –2
PART II: SHORT ANSWER (Each short answer question has a 1-point value!!)
31. Molarity is defined as the number of moles of solute per volume of solution in liters.
32. Ions that contain atoms of more than one element are called polyatomic ions.
33. Proton donors are known as Brønsted acids.
34. A compound that forms between a non-metal and a non-metal is a molecular compound.
35. Stoichiometric coefficients found in a balanced equation can be used to derive mole ratios.
36. When an element is oxidized, it loses electrons.
37. A neutralization reaction involves the reaction of a(n) acid with a(n) base.
38. A chemical equation without the spectator ions is called a net ionic equation.
PART III: NOMENCLATURE OF MOLECULAR AND IONIC COMPOUNDS 39. (4 pts) Name the following compounds:
a. Al2(SO4)3
Aluminum sulfate b. Cl2O5
Dichlorine pentaoxide
c. Li2HPO4
Lithium hydrogen phosphate d. Ba(ClO3)2
Barium chlorate
40. (4 pts) Write the correct formula for each of the following compounds:
a. Calcium Nitrite;
Ca(NO2)2
b. Copper(I) Cyanide;
CuCN
c. Sodium Nitride;
Na3N
d. Potassium Permanganate;
KMnO4 PART IV: CHEMICAL REACTIONS
41. (9 pts) Write (clearly and distinctly) the complete and net ionic equations for each of the following reactions IN WATER. If no reaction occurs, write NR.
a. ZnCl2 + Na2SO4 NO Reaction, All potential products are soluble ionic compounds
b. CuSO4 (aq) + Na2CO3 (aq) CuCO3 (s) + Na2CO3 (aq)
Cu2+ (aq) + SO42- (aq) + 2 Na+ (aq) + CO32- (aq) 2 Na+ (aq) + SO42- (aq) + CuCO3 (s)
Net: Cu2+ (aq) + CO32- (aq) (aq) CuCO3 (s)
c. CH3COOH (aq) + KOH (aq) CH3COOK (aq) + H2O (l) CH3COOH (aq) + K+ (aq) + OH-
(aq) CH3COO-
(aq) + K+ (aq) + H2O (l)
Net: CH3COOH (aq) + OH-
(aq) CH3COO-
(aq) + H2O (l)
42. (4 pts) Find which of the equations below represent redox reactions. Indicate the element that is reduced and the element that is oxidized. Write their oxidation numbers before and after the reaction.
a. NaNO3 (s) + Pb (s) NaNO2 (s) + PbO (s) redox reaction Oxidized: Pb; oxidation number changes from 0 to +2
Reduced: N; oxidation number changes from +5 to +4
b. Na2SO4 (s) + 4 C (s) Na2S (s) + 4 CO (s) redox reaction Oxidized: C; oxidation number changes from 0 to +2
Reduced: S; oxidation number changes from +6 to -2
PART V: CONCEPTS
43. (4 pts) The following diagram represents the reaction of A2 (gray spheres) with B2 (white spheres):
a. Write a balanced chemical equation for the reaction;
A2 + 3 B2 2 AB3 b. Identify the limiting reactant;
Limiting reactant: B2 Reactant in excess: A2
c. How many moles of product can be made from 1.0 mol of A2 and 1.0 mol of B2? 1 mol A2 x 2 mol AB3/1 mol A2 = 2 mol AB3
1 mol B2 x 2 mol AB3/3 mol B2 = 2/3 mol AB3
The maximum # of moles that can be prepared from 1mol A2 and 1 mol B2 (theoretical yield) is 2/3 mol AB3 PART VI: CALCULATION PROBLEMS (Show your work in its entirety. Do not provide just a single number!).
44. (7 pts) Potassium superoxide, KO2, is used in re-breathing gas masks to generate oxygen according to the equation:
4 KO2 (s) + H2O (l) KOH (s) + O2 (g) a. Balance the above equation;
4 KO2 (s) + 2 H2O (l) 4 KOH (s) + 3 O2 (g)
b. If a reaction vessel contains 0.25 mol of KO2 and 0.15 mol of H2O, find the following:
i. The limiting reactant in the process;
Answer: Examine separately the amounts of a product generated from KO2 and H2O individually. Since the next question is about the quantity of O2, let us choose O2 as the product to study:
1. Mol amount of O2 from KO2: 0.25 mol KO2 x (3 mol O2/4 mol KO2) = 0.19 mol O2 2. Mol amount of O2 from H2O: 0.15 mol H2O x (3 mol O2/2 mol H2O) = 0.22 mol O2 Therefore KO2 is the limiting reactant.
ii. The theoretical yield of O2 (in grams!);
Answer: In the previous part we found the theoretical yield of O2 in moles. All we have to do is convert it to grams:
0.19 mol O2 x (32.00 g O2/1 mol O2) = 6.1 g O2
iii. The excess amount of the reactant that is not the limiting one (in grams!);
Answer: The other reactant, H2O, is in excess. Let us start with finding the amount of water consumed in the reaction:
0.25 mol KO2 x (2 mol H2O/4 mol KO2) x (18.02 g H2O/1 mol H2O) = 2.3 g H2O was consumed The total amount of H2O (in grams) is: 0.15 mol H2O x (18.02 g H2O/1 mol H2O) = 2.7 g Therefore the excess amount is: 2.7 – 2.3 = 0.4 g of H2O
c. Suppose the process occurred with a 90% yield of oxygen. What was the actual yield of O2? Answer: It is based on the definition of percentage yield:
90% = (actual yield/theoretical yield) x 100 Therefore: Actual yield = (90 x 6.1)/100 = 5.5 g
45. (3 pts) BONUS PROBLEM (You do not have to solve this problem in order to get full credit. But in order to receive credit for this problem, it has to be solved entirely!!). Element X, a member of Group 5A, forms two chlorides XCl3 and XCl5. Reaction of an excess of Cl2 with 8.729 g of XCl3 yields 13.233 g of XCl5. What is the atomic mass and the identity of element X?
Solution: We are given the quantities of XCl3 and XCl5 and, in addition, we can write the balanced chemical equation:
XCl3 + Cl2 XCl5
According to that equation, the mole ratio is 1 mol XCl5/1 mol XCl3. We also need to realize that Molar Mass of XCl5 = (Molar Mass of XCl3 + Molar Mass Cl2) = (Molar Mass XCl3 + 70.90) = (Molecular weight XCl3 + 70.90) g/1 mol Let Molecular Weight XCl3 = M
Then Molar Mass XCl3 = M g XCl3/1 mol and Molar mass XCl5 = (M + 70.90) g XCl5/1 mol According to the balanced chemical equation:
8.729 g XCl3 x 1 mol XCl3/M g XCl3 x 1 mol XCl5/1 mol XCl3 x (M + 70.90) g XCl5/1 mol XCl5 = 13.233 g XCl5 We have obtained an equation with one unknown. Solving for M we find:
8.729 x (M + 70.90)/M = 13.233
M = 137.41
Thus, the molecular weight of XCl3 is M = 137.41 amu The combined weight of three Cl atoms is 30.45 x 3 = 106.35
The weight of element X = 137.41 – 106.35 = 31.06 Element X is Phosphorous (P).
