# Practice questions for Ch. 3

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1 Name: Class: Date: ID: A Practice questions for Ch A hypothetical element consists of two isotopes of masses amu and amu with abundances of 25.7% and 74.3%, respectively. What is the average atomic mass of this element? A) amu B) amu C) 70.5 amu D) 71.4 amu E) amu 2. Naturally occurring copper exists in two isotopic forms: 63 Cu and 65 Cu. The atomic mass of copper is amu. What is the approximate natural abundance of 63 Cu? A) 63% B) 90% C) 70% D) 50% E) 30% 3. The average mass of a carbon atom is Assuming you were able to pick up only one carbon unit, the chances that you would randomly get one with a mass of is A) 0% B) 0.011% C) about 12% D) % E) greater than 50% 4. Iron is biologically important in the transport of oxygen by red blood cells from the lungs to the various organs of the body. In the blood of an adult human, there are approximately red blood cells with a total of 2.90 g of iron. On the average, how many iron atoms are present in each red blood cell? (molar mass Fe = g/mol) A) B) C) D) E)

2 5. A sample of ammonia has a mass of 43.5 g. How many molecules are in this sample? A) 2.55 molecules B) molecules C) molecules D) molecules E) molecules 6. Phosphorus has the molecular formula P 4, and sulfur has the molecular formula S 8. How many grams of phosphorus contain the same number of molecules as 4.23 g of sulfur? A) 2.04 g B) g C) 4.08 g D) 4.23 g E) none of these 7. A given sample of a xenon fluoride compound contains molecules of a single type XeF n, where n is some whole number. Given that molecules of XeF n weigh g, calculate n. A) 1 B) 6 C) 4 D) 3 E) 2 8. How many atoms of hydrogen are present in 7.63 g of ammonia? A) B) C) D) E) One molecule of a compound weighs g. The molar mass of this compound is: A) 2.06 g/mol B) 567 g/mol C) 168 g/mol D) 176 g/mol E) none of these 2

3 10. A compound is composed of element X and hydrogen. Analysis shows the compound to be 80% X by mass, with three times as many hydrogen atoms as X atoms per molecule. Which element is element X? A) He B) C C) F D) S E) none of these 11. A substance contains 35.0 g nitrogen, 5.05 g hydrogen, and 60.0 g of oxygen. How many grams of hydrogen are there in a 153-g sample of this substance? A) 7.72 g B) 767 g C) 15.4 g D) 5.05 g E) 30.3 g 12. Chlorous acid, HClO 2, contains what percent hydrogen by mass? A) 1.92% B) 25.0% C) 23.4% D) 1.47% E) 5.18% 13. A substance, A 2 B, has the composition by mass of 60% A and 40% B. What is the composition of AB 2 by mass? A) 40% A, 60% B B) 50% A, 50% B C) 27% A, 73% B D) 33% A, 67% B E) none of these 14. Which of the following compounds has the same percent composition by mass as styrene, C 8 H 8? A) acetylene, C 2 H 2 B) benzene, C 6 H 6 C) cyclobutadiene, C 4 H 4 D) α-ethyl naphthalene, C 12 H 12 E) all of these 3

4 15. Suppose you are given the percent by mass of the elements in a compound and you wish to determine the empirical formula. Which of the following is true? A) You must convert percent by mass to relative numbers of atoms. B) You must assume exactly g of the compound. C) You must divide all of the percent by mass numbers by the smallest percent by mass. D) You cannot solve for the empirical formula without the molar mass. E) At least two of the above (A-D) are true. 16. A hydrocarbon (a compound consisting solely of carbon and hydrogen) is found to be 85.6% carbon by mass. What is the empirical formula for this compound? A) CH B) CH 2 C) C 2 H D) C 3 H E) CH 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 g/mol. How many atoms of carbon does a molecule of lindane contain? A) 2 B) 3 C) 4 D) 6 E) Balanced chemical equations imply which of the following? A) Numbers of molecules are conserved in chemical change. B) Numbers of atoms are conserved in chemical change. C) Volume is conserved in chemical change. D) A and B E) B and C 19. In balancing an equation, we change the to make the number of atoms on each side of the equation balance. A) formulas of compounds in the reactants B) coefficients of compounds C) formulas of compounds in the products D) subscripts of compounds E) none of these 4

5 20. What is the coefficient for oxygen when the following equation is balanced? NH 3 (g) + O 2 (g) NO 2 (g) + H 2 O(g) A) 3 B) 6 C) 7 D) 12 E) Potassium forms an oxide containing 1 oxygen atom for every 2 atoms of potassium. What is the coefficient of oxygen in the balanced equation for the reaction of potassium with oxygen to form this oxide? A) 0 B) 1 C) 2 D) 3 E) When the equation C 6 H 14 + O 2 CO 2 + H 2 O is balanced with the smallest set of integers, the sum of the coefficients is A) 4 B) 47 C) 15 D) 27 E) You heat g of a mixture of Fe 3 O 4 and FeO to form g Fe 2 O 3. The mass of oxygen reacted is A) g B) g C) g D) g E) none of these 24. When g of ethylene (C 2 H 4 ) burns in oxygen to give carbon dioxide and water, how many grams of CO 2 are formed? A) g B) g C) g D) 8.31 g E) g 5

6 25. Consider the following reaction: What mass of CCl 4 is formed by the reaction of 5.14 g of methane with an excess of chlorine? A) 12.3 g B) 0.54 g C) 791 g D) 49.3 g E) none of these 26. Nitric oxide, NO, is made from the oxidation of NH 3, and the reaction is represented by the equation: 4NH 3 + 5O 2 4NO + 6H 2 O What mass of O 2 would be required to react completely with 6.85 g of NH 3? A) 4.56 g O 2 B) 10.3 g O 2 C) 8.04 g O 2 D) 16.1 g O 2 E) 12.9 g O Phosphoric acid can be prepared by reaction of sulfuric acid with phosphate rock according to the equation: Ca 3 (PO 4 ) 2 + 3H 2 SO 4 3CaSO 4 + 2H 3 PO 4 Suppose the reaction is carried out starting with 129 g of Ca 3 (PO 4 ) 2 and 97.4 g of H 2 SO 4. Which substance is the limiting reactant? A) Ca 3 (PO 4 ) 2 B) H 2 SO 4 C) CaSO 4 D) H 3 PO 4 E) none of these 28. SO 2 reacts with H 2 S as follows: 2H 2 S + SO 2 3S + 2H 2 O When 7.50 g of H 2 S reacts with g of SO 2, which statement applies? A) 6.38 g of sulfur are formed. B) 10.6 g of sulfur are formed. C) moles of H 2 S remain. D) 1.13 g of H 2 S remain. E) SO 2 is the limiting reagent. 6

7 29. A 5.95-g sample of AgNO 3 is reacted with BaCl 2 according to the equation to give 3.17 g of AgCl. What is the percent yield of AgCl? A) 45.0% B) 53.3% C) 31.6% D) 63.1% E) 100% 30. Consider the following reaction: 2A + B 3C + D 3.0 mol A and 2.0 mol B react to form 4.0 mol C. What is the percent yield of this reaction? A) 50% B) 67% C) 75% D) 89% E) 100% 31. For the reaction N 2 (g) + 2H 2 (g) N 2 H 4 (l), if the percent yield for this reaction is 82.0%, what is the actual mass of hydrazine (N 2 H 4 ) produced when g of nitrogen reacts with 4.45 g of hydrogen? A) 35.7 g N 2 H 4 B) 24.0 g N 2 H 4 C) 30.0 g N 2 H 4 D) 29.2 g N 2 H 4 E) 28.9 g N 2 H Tellurium consists of 3 common isotopes. Half of tellurium atoms have a mass of 127, and another 0.45 of tellurium atoms weigh 128. What is the mass of the remaining isotope? The atomic mass of tellurium is A) 126 B) 127 C) 129 D) 130 E) 131 7

8 ID: A Practice questions for Ch. 3 Answer Section 1. ANS: D PTS: 1 DIF: Easy REF: 3.2 KEY: Chemistry general chemistry early atomic theory atomic theory of matter atomic weight mass spectroscopy 2. ANS: C PTS: 1 DIF: Moderate REF: 3.2 KEY: Chemistry general chemistry early atomic theory atomic theory of matter atomic weight mass spectroscopy 3. ANS: A PTS: 1 DIF: Easy REF: 3.2 KEY: Chemistry general chemistry early atomic theory atomic theory of matter atomic weight 4. ANS: B PTS: 1 DIF: Moderate REF: 3.3 KEY: Chemistry general chemistry stoichiometry mass and moles of substance mole 5. ANS: D PTS: 1 DIF: Moderate REF: 3.4 KEY: Chemistry general chemistry stoichiometry mass and moles of substance mole 6. ANS: A PTS: 1 DIF: Moderate REF: 3.4 KEY: Chemistry general chemistry stoichiometry mass and moles of substance mole 7. ANS: E PTS: 1 DIF: Moderate REF: 3.4 KEY: Chemistry general chemistry stoichiometry mass and moles of substance mole 8. ANS: D PTS: 1 DIF: Moderate REF: 3.4 KEY: Chemistry general chemistry stoichiometry mass and moles of substance mole 9. ANS: D PTS: 1 DIF: Moderate REF: 3.4 KEY: Chemistry general chemistry stoichiometry mass and moles of substance mole 10. ANS: B PTS: 1 DIF: Difficult REF: 3.6 KEY: Chemistry general chemistry stoichiometry determining chemical formulas mass percentage 11. ANS: A PTS: 1 DIF: Moderate REF: 3.6 KEY: Chemistry general chemistry stoichiometry determining chemical formulas 12. ANS: D PTS: 1 DIF: Easy REF: 3.6 KEY: Chemistry general chemistry stoichiometry determining chemical formulas 13. ANS: C PTS: 1 DIF: Moderate REF: 3.6 KEY: Chemistry general chemistry stoichiometry determining chemical formulas mass percentage 1

9 ID: A 14. ANS: E PTS: 1 DIF: Easy REF: 3.6 KEY: Chemistry general chemistry stoichiometry determining chemical formulas 15. ANS: A PTS: 1 DIF: Easy REF: 3.7 KEY: Chemistry general chemistry stoichiometry determining chemical formulas empirical formula 16. ANS: B PTS: 1 DIF: Easy REF: 3.7 KEY: Chemistry general chemistry stoichiometry determining chemical formulas mass percentage 17. ANS: D PTS: 1 DIF: Moderate REF: 3.7 KEY: Chemistry general chemistry stoichiometry determining chemical formulas molecular formula 18. ANS: B PTS: 1 DIF: Easy REF: 3.9 KEY: Chemistry general chemistry early atomic theory chemical equation balancing chemical equation 19. ANS: B PTS: 1 DIF: Easy REF: 3.9 KEY: Chemistry general chemistry early atomic theory chemical equation balancing chemical equation 20. ANS: C PTS: 1 DIF: Easy REF: 3.9 KEY: Chemistry general chemistry early atomic theory chemical equation balancing chemical equation 21. ANS: B PTS: 1 DIF: Moderate REF: 3.9 KEY: Chemistry general chemistry early atomic theory chemical equation balancing chemical equation 22. ANS: B PTS: 1 DIF: Easy REF: 3.9 KEY: Chemistry general chemistry early atomic theory chemical equation balancing chemical equation 23. ANS: A PTS: 1 DIF: Easy REF: 3.1 KEY: Chemistry general chemistry general concepts matter Law of Conservation of Mass 24. ANS: A PTS: 1 DIF: Moderate REF: 3.1 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation amounts of substances 25. ANS: D PTS: 1 DIF: Easy REF: 3.1 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation amounts of substances 26. ANS: D PTS: 1 DIF: Easy REF: 3.1 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation amounts of substances 27. ANS: B PTS: 1 DIF: Easy REF: 3.11 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation limiting reactant 2

10 ID: A 28. ANS: B PTS: 1 DIF: Moderate REF: 3.11 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation limiting reactant 29. ANS: D PTS: 1 DIF: Moderate REF: 3.11 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation limiting reactant 30. ANS: D PTS: 1 DIF: Moderate REF: 3.11 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation limiting reactant 31. ANS: B PTS: 1 DIF: Difficult REF: 3.11 KEY: Chemistry general chemistry stoichiometry stoichiometry calculation limiting reactant 32. ANS: D PTS: 1 DIF: Moderate REF: 3.2 KEY: Chemistry general chemistry early atomic theory atomic theory of matter atomic weight 3

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