Review Assignment #3

Chapter 3: Stoichiometry

46.

a. find mole ratios relative to oxygen:

Therefore the empirical formula is C₂H₆O.

b.

 

 

 

So, the empirical formula is FeO_1.5, (yuck), or preferably, Fe₂O₃.

c. For problems that provide mass percentages, assume you start with a mass of 100g.

 
Therefore the empirical formula is CH₂O.

52. a)

 

Find ratios relative to oxygen:
 
 

Therefore the empirical formula is C₄H₅N₂O.

The empirical formula weight is 97.1. So the number of formula units per molecule is given by:
 

Therefore, there are two "C₄H₅NO₂ units" per molecule of caffeine, so its molecular formula is C₈H₁₀N₄O₂.
 

b)

 

Find ratios relative to sodium:
 

 

Therefore the empirical formula is NaC₅H₈NO₄.

The empirical formula weight is 169.1. Since this is equivalent to the molar mass, the empirical formula and the molecular formulas are the same in this case.

a)

 

b)

 
 

c)

66. The balanced equation is:

 

The sum of all the masses of the products is: 0.999 g. The small deviation from 1.00 is due to rounding errors in the calculations, but the result is in accord with The Law of Conservation of Mass.

74. The balanced equation is:

To determine which is the limiting reactant, calculate the mass of product possible based on the masses of each reactant individually.
 

 

 

Since complete reaction of O₂ yields a smaller amount of product than does complete reaction of NH₃, the O₂ is the limiting reactant. Only 2.14 g of NO form.
 

The amount of excess NH₃ can be found by calculating the amount of NH₃ consumed and then finding the difference from the original mass:
 

 

NH₃ remaining: 2.50 g - 1.21 g = 1.29 g NH₃.

78. The balanced equation for this reaction is:

This is a limiting reactant problem.

 

Thus, AgNO₃ is the limiting reactant and will be completely consumed. 4.06 g of Ag₂CO₃ will be formed. The amount of NaNO₃ that forms is:

 
 

The amount of Na₂CO₃ that reacts is:
 
 

 
 

Therefore, the amount of Na₂CO₃ that remains when the reaction is complete is: 5.00 g - 1.56 g = 3.44 g. So, in summary, the masses of the salts are:

90. This is a challenging problem that requires you to apply the mole concept in a creative way. The balanced reaction is:

 
From the stoichiometry, we know that the number of moles of XI3 is equal to the number of moles of XCl3. We also know the masses that correspond to each of these materials. But, we do not know the actual number of moles because we don't know the molar mass. We can, however, express the molar masses the following way:

molar mass XI₃ = [x + 3(126.9)] g/mol

molar mass XCl₃ = [x + 3(35.45)] g/mol

We can then write a stoichiometric relationship in the usual way:
 

 

0.500 x + 53.2 = 0.236 x + 89.84

0.264 x = 36.64

x = 138.8

This is the atomic weight of element X; X is therefore Lanthanum!

97. The balanced chemical reaction is:

107. There are two reactions at work here:
 
 

The mass of sulfur burned each day is: (2000 tons) (0.025) = 50 tons.

Setting up the stoichiometric relationships yields:

 
 

10      a)

b)

18. a) This is an example of Boyle's Law:

P₁V₁ = P₂V₂

 

b) This is an example of Charles' Law:

 

 

 
a)

 

b)

 

c)

 

d)

 

30.    a)

 

b) STP = 1.00 atm, 273 K.

 

c) 130° F = 54° C = 327 K

41. To determine how much magnesium will react, we must first determine how many moles of O₂ are present.

50.

 

 

 

The total pressure is: P_total = P_CH4 + P_C2H4 + P_C4H10 = 4.54 atm

54.

 

 

 

 
 

b)

 

 

56.

 

 

P_total = P_N2 + P_SO2 = 0.153 atm + 0.131 atm = 0.284 atm
 
 

 

 

 

So, I₂ is the limiting reactant and 17.5 g of IF₅ will form.

The partial pressure of IF₅ is:

 

To find the mole fraction, the number of moles of F₂ that remain needs to be determined:

 

The mole fraction is then:
 

 

Chapter 11: Phase Changes

44a) 1.4 atm is equal to 1064 torr (pardon the significant figures violation). The temperature at which the vapor pressure of water is approximately equal to this pressure is about 110° C.

b) Ethyl alcohol will boil at 60° C when the external pressure is approximately 350 torr.
 

46. Water will boil in Reno at approximately 97° C.

52c. O₂ will not sublime at 1 atm; the liquid phase is stable over a well defined temperature range.