Cis and trans isomers

Do cis- and trans-isomers of the same compound have different molecular weights?

nope...
... .. .H.. H
... ....|....|
CH3-C=C-CH3... cis

... .. .H
... ....|
CH3-C=C-CH3... trans
... ... ... |
... .... ...H

both are C4H8 with molar mass 4x12 + 8x1 = 56 g/mole

Nope, they have the same molecular formula, thus the same molecular weight.

Your friend,
Zen Jesus