Explanation:
The mass spectrum of C2F6 shows the abundance of different isotopes present in the compound. It is important to note that isotopes have the same atomic number but differ in their mass number.

Isotopic Abundances:
- 12C: 98.89%
- 13C: 1.11%
- 19F: 100%

Calculating Heights of Peaks:
- The peak at mass number 138 is due to the presence of two 19F isotopes and four 12C isotopes, as shown below:
- C2F6 → (12C)2(19F)6 → 4(12) + 2(19) = 138
- The peak at mass number 139 will be due to the presence of one 13C isotope and five 12C isotopes, as shown below:
- C2F6 → (12C)2(19F)5(13C)1 → 5(12) + 1(13) + 2(19) = 139
- The peak at mass number 140 will be due to the presence of two 13C isotopes and four 12C isotopes, as shown below:
- C2F6 → (12C)2(19F)4(13C)2 → 4(12) + 2(13) + 2(19) = 140

Calculating Heights of Peaks:
- Peak at mass number 139:
- The peak at mass number 138 is 100 units tall
- The abundance of 12C in C2F6 is 98.89%
- The abundance of 13C in C2F6 is 1.11%
- Therefore, the height of the peak at mass number 139 will be:
- (5/2) * (1.11/98.89) * 100 = 2.78 units
- Peak at mass number 140:
- The peak at mass number 138 is 100 units tall
- The abundance of 12C in C2F6 is 98.89%
- The abundance of 13C in C2F6 is 1.11%
- Therefore, the height of the peak at mass number 140 will be:
- (2/2) * ((1.11/98.89) * (1.11/98.89)) * 100 = 0.05 units

Conclusion:
The height of the peak at mass number 139 will be 2.78 units and the height of the peak at mass number 140 will be 0.05 units. This calculation shows that the abundance of isotopes in a compound can affect the height of peaks in its mass spectrum.