Samples: Free-Response Questions
A Note to Students: Part A of Section II of the AP chemistry exam is always an equilibrium
problem. Part A consists of one equilibrium question, and you must do this question; there
is no choice among questions. This one question is worth 20% of the grade for Section II.
Needless to say, your score on this one question is very important to your success on the
AP chemistry exam.The equilibrium problem will represent one of three possible types:
Gaseous equilibrium — Kcor Kp
Acid-base equilibrium — Kaor Kb
Solubility — Ksp- 250.0 grams of solid copper(II) nitrate is placed in an empty 4.0-liter flask. Upon heating
the flask to 250°C, some of the solid decomposes into solid copper(II) oxide, gaseous
nitrogen(IV) oxide, and oxygen gas. At equilibrium, the pressure is measured and found
to be 5.50 atmospheres.
(a) Write the balanced equation for the reaction.
(b) Calculate the number of moles of oxygen gas present in the flask at equilibrium.
(c) Calculate the number of grams of solid copper(II) nitrate that remained in the flask at
equilibrium.
(d) Write the equilibrium expression for Kpand calculate the value of the equilibrium
constant.
(e) If 420.0 grams of the copper(II) nitrate had been placed into the empty flask at
250 °C, what would the total pressure have been at equilibrium?Answer
- Given: 250.0 g Cu(NO 3 ) 2 in 4.0-liter flask
Heated to 250°C, reaches equilibrium
Total pressure at equilibrium = 5.50 atmospheres
(a) Restatement: Balanced reaction.
2 Cu(NO 3 ) 2 (s) ↔2 CuO(s) + 4 NO 2 (g) + O 2 (g)Equilibrium