AP Chemistry:  Practice Gas Equilibrium Problems          February 2003

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1988 A

At elevated temperatures, SbCl₅ gas decomposes into SbCl₃ gas and Cl₂ gas as shown by the following equation: SbCl₅(g) « SbCl₃(g) + Cl₂(g)

(a)     An 89.7 gram sample of SbCl₅ (molecular weight 299.0) is placed in an evacuated 15.0 litre container at 182ºC.

i.      What is the concentration in moles per litre of SbCl₅ in the container before any decomposition occurs?

                   ii.     What is the pressure in atmospheres of SbCl₅ in the container before any decomposition occurs?

(b)     If the SbCl₅ is 29.2 percent decomposed when equilibrium is established at 182ºC, calculate the value for either equilibrium constant K_p or K_c, for this decomposition reaction. Indicated whether you are calculating K_p or K_c.

(c)     In order to produce some SbCl₅, a 1.00 mole sample of SbCl₃ is first placed in an empty 2.00 litre container maintained at a temperature different from 182ºC. At this temperature, K_c, equals 0.117. How many moles of Cl₂ must be added to this container to reduce the number of moles of SbCl₃ to 0.700 mole at equilibrium?

1981 A

Ammonium hydrogen sulfide is a crystalline solid that decomposes as follows:

NH₄HS(s) « NH₃(g) + H₂S(g)

(a)     Some solid NH₄HS is placed in an evacuated vessel at 25ºC. After equilibrium is attained, the total pressure inside the vessel is found to be 0.659 atmosphere. Some solid NH₄HS remains in the vessel at equilibrium. For this decomposition, write the expression for K_P and calculate its numerical value at 25ºC.

(b)     Some extra NH₃ gas is injected into the vessel containing the sample described in part (a). When equilibrium is reestablished at 25ºC, the partial pressure of NH₃ in the vessel is twice the partial pressure of H₂S. Calculate the numerical value of the partial pressure of NH₃ and the partial pressure of H₂S in the vessel after the NH₃ has been added and the equilibrium has been reestablished.

(c)     In a different experiment, NH₃ gas and H₂S gas are introduced into an empty 1.00 liter vessel at 25ºC. The initial partial pressure of each gas is 0.500 atmospheres. Calculate the number of moles of solid NH₄HS that is present when equilibrium is established.

1983 A

Sulfuryl chloride, SO₂Cl₂, is a highly reactive gaseous compound. When heated, it decomposes as follows: SO₂Cl_2(g) ® SO₂(g) + Cl₂(g). This decomposition is endothermic. A sample of 3.509 grams of SO₂Cl₂ is placed in an evacuated 1.00 litre bulb and the temperature is raised to 375K.

(a)     What would be the pressure in atmospheres in the bulb if no dissociation of the SO₂Cl_2(g) occurred?

(b)     When the system has come to equilibrium at 375K, the total pressure in the bulb is found to be 1.43 atmospheres. Calculate the partial pressures of SO₂, Cl₂, and SO₂Cl₂ at equilibrium at 375K. 

(c)     Give the expression for the equilibrium constant (either K_p or K_c) for the decomposition of SO₂Cl_2(g) at 375K. Calculate the value of the equilibrium constant you have given, and specify its units.

(d)     If the temperature were raised to 500K, what effect would this have on the equilibrium constant? Explain briefly.  Hint:  At the top of the page, it tells you that the reaction is endothermic.

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