AQA Physics A2 - Textbook Futher Practice Question Help

Which part are you stuck on and what are you you finding difficult?

I've proved Part i but not sure how to approach part ii of Question 1. Textbook answers also cannot be found for the further practice questions.

How do I calculate the Mass of the Oxygen atom if I don't know the number of moles.

For (a)(ii)

GPE “of an oxygen molecule” is given by mass of an oxygen molecule × gravitational potential at that point.

You don't need to know the number of moles of oxygen molecules because you are given the molar mass of oxygen which is the mass of oxygen molecules in one mole.
You should also know that Avogadro Constant is 6.02 × 10²³ particles/mol.

Mass of one oxygen molecule = molar mass of oxygen × Avogadro Constant