A level chemistry molecular formula question - please help!!

Hi everyone - I was wondering if anyone would be able to explain this question to me? It's from the AQA specimen paper 2 (set 1), and I can't work out how to get the molecular formula from it...

"03.8) Isooctane reacts with an excess of chlorine to form a mixture of chlorinated compounds.
One of these compounds contains 24.6% carbon and 2.56% hydrogen by mass.
Calculate the molecular formula of this compound." (3 marks)

I've managed to get to the empirical formula (C4H5Cl4), but from there the mark scheme just says "Mr = C8H10Cl8" with no explanation of how they got there! An Mr of the actual compound hasn't been given (as usually I'd divide that by the Mr of the empirical formula to get the molecular formula), so I'm at a bit of a loss.

Any help would be greatly appreciated!!

Idk if this is right because I do OCR, but I think reactions with chlorine and an alkane (Isooctane in this case) are always substitution reactions and therefore don't break down the chain of carbons. So in isooctane there are 8 carbons and the product after reacting with excess chlorine must also have 8 carbons.