# A Level Chemistry 2017 Paper 1 Question 11 Solution - Exclusive

In this exclusive video we want to discuss the suggested solution for A Levels Chemistry (H2 Chemistry) 2017 Paper 1 Question 11.

Let's take a look at this question:

The topic tested in this question is on Kinetics.

Now we know that the half-life for first order reactions is a constant.

So for this question, the half-life of 30 minutes will be a constant.

If we start off with 1 mol of X_{2}, we can figure out that after 2 half-lives, we are left with 25% of X_{2}.

This means that 75% of X_{2} has been converted to products or X atoms, which corresponds to what the question wants.

Now we can go through the options and see if the following statements are true.

**Statement One**

Since two half-lives have passed and each half-life is 30 minutes, total time elapsed is 60 minutes.

So statement one is true.

**Statement Two**

Since 75% of X_{2} has been converted, 0.75 mol of X_{2} has been converted to form 1.5 mol of X atoms.

So statement two is true.

**Statement Three**

The number of moles of remaining X_{2} is 0.25 while the number of moles of X atoms formed is 1.5 mol.

Thus the total number of moles is 1.75 moles.

At constant temperature and volume, we can use the Ideal Gas Equation to deduce that the pressure is proportionate to number of moles.

This means that:

- initially when number of moles of gas is 1, pressure is p

- finally when number of moles of gas is 1.75, pressure is 1.75p which works out to be 7p/4

Therefore statement three is also true.

Finally, looking back at the options A to D, A must be the answer since all three statements are true.

Check out this video for the full solution and detailed calculation!

Topic: Kinetics, Physical Chemistry, A Level Chemistry, Singapore

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