Boiling points of compounds

Which of the following pairs of compounds has the higher boiling point and give a brief reason for your answer.

i) CH4 and SiH4

ii) C3H8 and CH3Cl

I know that the bigger the molecules in the compounds the higher the attraction and therefore the higher the boiling point but have no idea how to figure out which one of these is the bigger of the two molecule.

Any help is appriciated

Thanks

Reply 1

devilschild

KaosLisa

well for i) silicon has a greater mass than carbon so it should be SiH4 that has the greater boiling point due to increased Van der Waals forces between molecules.
as for ii) i think it'll be CH3Cl with the greater boiling point cos it has a greater mass (50.5 compared to 44) and also contains Cl so will have a permanent dipole so there will be increased attraction due to permanent dipole forces and more energy will be required to separate the molecules

To figure out which is the bigger of the 2 molecules just calculate relative formula mass of each molecule- add up the masses of each atom they contain.