natural abundance of C12 and C13 isotopes of carbon are 99% and 1% respectively.assuming they only contribute to the molar weight of C2F4 ,what is the percentage of C2F4 having molar mass 101?

In molecular formula of C2F4 there are 4 F atoms. F has atomic mass of 19 so 4 F would have atomic mass of 76. 2 C would have atomic mass of either 24(12C) or 26 (13C). So the possible molar mass of C2F4 are 100(76+12+12), 102(76+13+13) or 101(76+12+13).
Now according to % abundance 
Percentage of C2F4 of molar mass 100(when both the C are 12C) = 1/100 x 1/100 x 100 = 0.01 %
Percentage of C2F4 of molar mass 102 (when both the C are 13C)= 99/100 x 99/100 x 100 = 98.01%
Percentage of C2F4 of molar mass 101 = 100-(98.01+0.01) =1.98 %

 

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