Please help with this reactivity of mass question.

What mass of oxygen is formed when 735 g of potassium chlorate decomposes?

2KClO3 -> 2KCl + 3O2

So first you need to divide 735, by the relative formula mass.

Once you've done that you can multiply it by the relative mass of all the oxygens in the molecule.

S/he'll also need to take note of the 2:3 ratio.

No because there is only 6 molecules of oxygen in the whole reaction

molecules? did you mean atoms?

n(KClO₃) = m / M_r = 735 / 122.5 (using a GCSE Periodic Table*) = 6 mol (*in which case I won't bother with sig fig)

n(O₂) = n(KClO₃) x 3/2 = 6 x 1.5 = 9 mol

m(O₂) = n x M_r = 9 x 32 = 288 g

Why are you involving moles?
If there's no oxygen lost during the equation, surely you just have to divide by relative formula mass ~ 122.5

And there's two of them so: 735/245 = 3
So then you multiply: 3*3*2 = 18
Multiply that by the relative mass of oxygen: 18*16 = 288g

You don't have to do anything to do with moles :c

The balanced equations shows the ratios of which molecules react and are created.

So in this instance, for every 2 KClO₃ molecules that react, 3 KCl molecules are made along with 3 O₂ molecules.
This means that for example if 2 moles of KClO₃ reacted you would get, 2 moles of KCl and 3 moles of O₂
(Remember that moles are a specific amount of something)

Meaning that, as Pigster said, the OP would need to consider the 2:3 ratio of the KClO₃ and O₂ to find the correct number of moles of Oxygen and thus mass.

That is 100% correct, but I got the question right, without using moles at all, so it's not required for this question?

The calculation you used is perfectly acceptable at GCSE (and I suspect AS), but is not good enough for A level. It isn't enough to get the correct answer (well, actually it depends on which exam board you do and what the mark scheme expects), you also have to show the correct working.

Structured questions are going to be a much smaller feature of most chemistry exams, these days, but if the Q asks you to work out the amount of something, you have to be able to.

Your advice was understandable to you, but I would say was misleading to the OP

"So first you need to divide 735, by the relative formula mass."

The RFM of KClO3 is as you say ~ 122.5, but later you then added that this value needed multiplying by 2.

My method and my layout of my answer was (hopefully) clearer.

The advice I give to my students (as an examiner myself) is to make your answer as easy to follow as possible to make it as easy as possible for the marker to award the marks. This is especially important if you make a stupid mistake, such as to write 882 rather than 288 (I see transcription errors all the time).

With my layout, the answer 882 would only lose 1 mark. I am not certain if your layout would score any marks for your working.