Natural abundances of 12C and 13C isotopes of carbon are 99% and 1%, respectively. Assuming they only contribute to the mol. wt. of C2F4, the percentage of C2F4 having a molecular mass of 101 is:
A
1.00
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B
98
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C
0.198
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D
99
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Solution
The correct option is A1.00 The molecular mass of C2F4 containing C-12 is 2(12)+4(19)=100 g/mol The molecular mass of C2F4 containing one C-12 and one C-13 is 12+13+4(19)=101 g/mol Natural abundances of C-12 and C-13 isotopes of carbon are 99% and 1%, respectively. 100 C2F4 molecules will have 200 C atoms out of whic 1% or 2 C atoms will be C-13. Assuming that these two C-13 atoms will be in different C2F4 molecules, the percentage of C2F4 having a molecular mass of 101 is 2200×100=1 %