## Assignment—Composition of Substances and Solutions

As you work these matter and measurement problems, consider and explain:

A. What type of question is it?

B. How do you know what type of question it is?

C. What information are you looking for?

D. What information do they give?

E. How will you go about solving this?

F. Show how to solve the problem.

G. Be able to answer for a different reaction, number, set of conditions, etc.

## Sample Questions

1. The atomic mass of rhenium is 186.2. Given that 37.1% of natural rhenium is rhenium-185, what is the other stable isotope?
2. Naturally occurring element X exists in three isotopic forms: X-28 (27.977 amu, 92.23% abundance), X-29 (28.976 amu, 4.67% abundance), and X-30 (29.974 amu, 3.10% abundance). Calculate the atomic weight of X.
3. The average mass of a carbon atom is 12.011. Assuming you were able to pick up only one carbon unit, what are the chances that you would randomly get one with a mass of 12.011?
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 2.64 × 1013 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 = 55.85 g/mol)
5. A sample of ammonia has a mass of 43.5 g. How many molecules are in this sample?
6. What is the molar mass of ethanol (C2H5OH)?
7. Roundup, an herbicide manufactured by Monsanto, has the formula C3H8NO5P. How many moles of molecules are there in a 295.1-g sample of Roundup?
8. Phosphoric acid can be prepared by reaction of sulfuric acid with “phosphate rock” according to the equation:
Ca3(PO4)2 + 3H2SO4 → 3CaSO4 + 2H3PO4
How many oxygen atoms are there in 1.75 ng of Ca3(PO4)2?
9. What is the mass of a 6.761-mol sample of sodium hydroxide?
10. One molecule of a compound weighs 2.93 × 10–22 g. What is the molar mass of this compound?
11. How many grams of potassium are in 27.8 g of K2CrO7?
12. Chlorous acid, HClO2, contains what percent hydrogen by mass?
13. The mineral vanadinite has the formula Pb5(VO4)3Cl. What mass percent of chlorine does it contain?
14. The molar mass of an insecticide, dibromoethane, is 187.9 g/mol. Its molecular formula is C2H4Br2. What percent by mass of bromine does dibromoethane contain?
15. A chloride of rhenium contains 63.6% rhenium. What is the formula of this compound?
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?
17. The empirical formula of styrene is CH; its molar mass is 104.1 g/mol. What is the molecular formula of styrene?
18. 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. What is the coefficient for water when the following equation is balanced?
As(OH)3(s) + H2SO4(aq) → As2(SO4)3(aq) + H2O(l)
20. What is the coefficient for oxygen when the following equation is balanced?
NH3(g) + O2(g) → NO2(g) + H2O(g)
21. Determine the coefficient for O2 when the following equation is balanced in standard form (smallest whole numbers).
C8H18(g) + O2(g) → CO2(g) + H2O(g)
22. When is this equation properly balanced?
wPCl5 + xH2O → yPOCl3 + zHCl
23. Give (in order) the correct coefficients to balance the following reaction: H2SnCl6 + H2S → SnS2 + HCl
24. A reaction occurs between sodium carbonate and hydrochloric acid producing sodium chloride, carbon dioxide, and water. What is the correct set of coefficients, respectively, for the balanced reaction?
25. You heat 3.869 g of a mixture of Fe3O4 and FeO to form 4.141 g Fe2O3. What is the mass of oxygen that reacted?
26. What would be the g Al / mole S ratio for the product of a reaction between aluminum and sulfur?
27. How many grams of Ca(NO3)2 can be produced by reacting excess HNO3 with 6.33 g of Ca(OH)2?
28. Nitric oxide, NO, is made from the oxidation of NH3, and the reaction is represented by the equation: 4NH3 + 5O2 → 4NO + 6H2O What mass of NO can be produced from 7.55 g of NH3?
29. For the reaction P4O10(s) + 6H2O(l) → 4H3PO4(aq), what mass of P4O10 must be consumed if 3.71 → 1023 molecules of H2O are also consumed?
30. The limiting reactant in a reaction
A) has the lowest coefficient in a balanced equation
B) is the reactant for which you have the fewest number of moles
C) has the lowest ratio of moles available/coefficient in the balanced equation
D) has the lowest ratio of coefficient in the balanced equation/moles available
E) none of these
31. Phosphoric acid can be prepared by reaction of sulfuric acid with “phosphate rock” according to the equation:
Ca3(PO4)2 + 3H2SO4 → 3CaSO4 + 2H3PO4
Suppose the reaction is carried out starting with 129 g of Ca3(PO4)2 and 97.4 g of H2SO4. Which substance is the limiting reactant?
32. A 15-g sample of lithium is reacted with 15 g of fluorine to form lithium fluoride: 2Li + F2 → 2LiF. After the reaction is complete, what will be present?
A) 2.16 moles lithium fluoride only
B) 0.789 moles lithium fluoride only
C) 2.16 moles lithium fluoride and 0.395 moles fluorine
D) 0.789 moles lithium fluoride and 1.37 moles lithium
E) none of these
33. 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?
34. If 45.0 g of O2 are mixed with 45.0 g of H2 and the mixture is ignited, what mass of water is produced?

1. ${}_{75}^{187}\text{Re}$

2. 28.09 amu
3. 0%
4. 1.18 × 109
5. 1.54 × 1024
6. 46.07 g/mol
7. 1.745
8. 2.72 × 1013
9. 270.4 g
10. 176 g/mol
11. 8.98 g
12. 1.47%
13. 2.50%
14. 85.05%
15. ReCl3
16. CH2
17. C8H8
18. B
19. 6
20. 7
21. 25
22. w = 1, x = 1, y = 1, z = 2
23. 1, 2, 1, 6
24. 1 2 2 1 1
25. 0.272 g
26. 17.99 g Al / mol S
27. 14.0 g
28. 13.3 g NO
29. 29.1 g P4O10
30. C
31. H2SO4
32. D
33. 89%
34. 50.7 g