A sealed container of gas at 20°C is heated until its volume doubles while pressure stays constant. What must the final temperature be?
A40°C
B313°C
C40 K
D293 K
Charles's law (V ∝ T at constant P) requires absolute temperature. 20°C = 293 K. To double volume, temperature must double to 586 K — which is 313°C, not 40°C. Using Celsius directly is the classic mistake; doubling 20°C to 40°C would only increase volume by about 7%, not double it.
Question 2 True / False
According to the ideal gas law, if you double both the pressure and the absolute temperature of a fixed amount of gas, the volume stays the same.
TTrue
FFalse
Answer: True
From PV = nRT, V = nRT/P. If both T and P double, the ratio T/P is unchanged, so V is unchanged. This is a non-obvious result that highlights why all four variables must be considered together.
Question 3 Short Answer
Why does the ideal gas law become inaccurate for real gases at very high pressures or very low temperatures?
Think about your answer, then reveal below.
Model answer: At high pressures, gas molecules are packed close enough that their own volume is no longer negligible, and intermolecular attractive forces become significant — both effects the ideal gas model ignores.
The two core assumptions of the ideal gas model are that molecules occupy zero volume and exert no forces on each other. These hold well when molecules are far apart (low pressure, high temperature), but break down when molecules are crowded together.