Chemistry Help - Empirical Formulas and Relative Abundance.

[Vampenstine]

It would be greatly appreciated if someone could try and explain some stuff to me, 'cause I haven't the foggiest idea of how to do my chemistry assignment that's due for tomorrow...

The measured average atomic weight of an element is 41.300 g mol?1. If its two
stable isotopes weigh 35.000 g mol?1 and 40.000 g mol?1, what are their relative abundances
expressed as percentages?

8.000 g of aluminium metal was reacted with 12.00 g of Cl2 gas, to form a compound X
containing only aluminium and chlorine. The aluminium was found to be in excess, and
2.471 g of un-reacted Al metal was recovered from the reaction. What is the empirical
formula of X?

That's the kind of calculations I can't do.
I need to get my head around how to calculate relative abundances and empirical formulas, the rest I should be alright with.

Anyone good enough at chemistry to try and explain?