3題普化問題請教各位~~拜託了!

1. Lithium has two stable isotopes with masses of 6.01512 amu and 7.01600 amu. The averge molar mass of Li is 6.941 amu. What is the percent abundance of each isotope?答案為何是 7.493% Li-6 and 92.51% Li-72.A 4.236g sample of a hydrocarbon is combusted to give 3.810g of H2O and 13.96g of CO2. What is the empirical formula compound?3.Calcium reacts with fluorine gas to produce calcium fluoride. How many moles of Ca will react with 1.0 moles of F2?拜託高手幫我解答!!拜託附上公式歐!!謝謝!!
1.設6.015(Li-6)有X%

7.016(Li-7)有(100-X)%平均[6.01512*X 7.016(100-X)]/100=(6.01512X 701.6-7.016X)/100=(701.6-1.00088X)/100=7.016-0.0100088X= 6.9410.0100088X=0.075

X= 7.493%所以Li-7有92.5066%2.H2O=18；CO2=44所以H2O有3.810/18= 0.212莫耳；H有0.423莫耳　　CO2有13.96/44=0.317莫耳；C有0.317莫耳C:H=0.317:0.423=3:4

故分子式為C3H43.Ca F2--