MCAT General Chemistry Question 111: Answer and Explanation

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Question: 111

6. An 8.00 g sample of NH₄NO₃ (s) is placed into an evacuated 10 L flask and heated to 227°C. After the NH₄NO₃ completely decomposes, what is the approximate pressure in the flask?

NH₄NO₃ (s) → N₂O (g) + H₂O (g)

A. 0.410 atm
B. 0.600 atm
C. 0.821 atm
D. 1.23 atm

Correct Answer: D

Explanation:

The first thing to do is balance the given chemical equation: NH₄NO₃ (s) → N₂O (g) + 2 H₂O (g). The mass given is 8.00 g, which represents 0.1 mol NH₄NO₃ (). When 0.1 mol of the solid decomposes, it will form 0.1 mol N₂O and 0.2 mol water. This gives approximately 0.3 moles of gas product. The ideal gas equation can be used to obtain the pressure in the flask:

Choice (C) is the result if one assumes the equation is balanced, obtaining 0.2 mol gas as the product.

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