# 2020 P1 Q6 - Compare PV against V Graph of Equal Masses of Gases

Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 6. We are required to determine which graph shows the results for neon.

Let us first consider the shape of PV against V graph for an ideal gas. From Ideal Gas Equation

PV = nRT

Gas constant R and temperature T are constants and for each gas, number of moles n will also be constant.

This means nRT is a constant and we can write the Ideal Gas Equation as

PV = nRT = k

The graph of PV against V will be a horizontal line, and the y-intercept will be k.

Next let us consider different gases of equal mass. A gas with a smaller molar mass will have a greater number of moles.

Since y-intercept k = nRT, larger n means k will also be larger.

This means a gas with smaller molar mass will have larger y-intercept k. We can now compare the molar mass of the gases given and determine which graph is neon.

CH4 has the smallest molar mass of 16.0 hence largest n and k that corresponds to graph A.

Ne has the next smallest molar mass of 20.2 hence second largest n and k that corresponds to graph B.

Cl2 has the largest molar mass of 71.0 hence its moles and k will be the smallest and that would correspond to graph D.

Therefore the answer to this question will be option B.

Topic: Gaseous State, Physical Chemistry, A Level Chemistry, Singapore

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