If two compounds have the same empirical formula but different molecular formulae, they must have

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Same empirical formula means the same atom ratio, hence the same percentage composition. Different molecular formulae means a different multiplier  — and  scales the molecular weight.

For example  () and  () share the empirical formula but differ in molecular weight ( vs ).

So options (a) and (d) are ruled out (percentage composition and vapour density are tied to the empirical formula and stay the same), (c) is unrelated — what must differ is the molecular weight.

Watch out: vapour density i

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