The mass spectrum of ethane, C2H6, has the following peaks and intensities. If t

Question

The mass spectrum of ethane, C2H6, has the following peaks and intensities. If the fragment ions that produce these peaks have formula C2H5+ , there is only one isotope of H present (1H, mass =1.0078 amu) and C has two naturally occurring isotopes, calculate from the given collected data Mass (amu) Abundance (counts) 29.0390 13438.0 30.0424 291.0 31.0458 2.0 a) the masses and relative abundances of each of the two naturally-occurring C isotopes. b) the weighted average atomic mass of C.

Mass (amu)

Abundance (counts)

29.0390

13438.0

30.0424

291.0

31.0458

2.0

   a)   the masses and relative abundances of each of the two naturally-occurring C isotopes.

   b)    the weighted average atomic mass of C.

Please show work.

Mass (amu)

Abundance (counts)

29.0390

13438.0

30.0424

291.0

31.0458

2.0

Explanation / Answer

a. Total no. of counts = 13438 + 291 + 2 = 13731

Mass of H = 1.0078 amu

Mass of 5H = 5.039 amu

Let the two C-isotopes be xC and yC.

Let mass of xC2H5 be 29.039 amu and mass of yC2H5 be 31.0458 amu. The mass with 291 counts could be from either of the signals.

So, 2xC = 29.039 - 5.039 = 24

xC = 12, so x = 12 amu

Relative abundance of 12C isotope is

RA1 = 13438 / 13731 = 97.8661 %

Now,

2yC = 31.0458 - 5.039

yC = 13.0034, so y = 13.0034 amu

Relative Abundance of 13.0034C isotope is

RA2 = 2 / 13731 = 0.0146 %

b. Weighted average atomic mass of C is

A = 0.000146 (13.0034) + 0.978661 (12) + 0.021193 (12.5017)

A = 12.0108 amu

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.