Suppose that, on a GC column, methyl hexanoate eluted in 1.87 ...

[vinney12]

Suppose that, on a GC column, methyl hexanoate eluted in 1.87 minutes, methyl heptanoate
  eluted in 2.35 minutes, and methyl octanoate eluted in 3.0 minutes. What is the structure of an
  unknown methyl ester, known to belong to the same homologous series, that elutes at 4.7 minutes?

Question 2

Helium is the carrier gas of choice for a gas chromatograph containing a thermal conductivity
  detector, and nitrogen gas is preferred for a gas chromatograph having a flame ionization
  detector. Suggest a reason for this.

[at]

Answer to Question 1

Construct a graph of the log of the retention time versus the number of carbons for the known
compounds. From the graph, read the number of carbons for the unknown methy ester, which,
in this case is methyl decanoate.

Answer to Question 2

Helium has a very high thermal conductivity, thus maximizing the response when compounds
of low thermal conductivity pass the detector. In addition, all organic compounds show a peak
response that rises above (rather than falls below) the baseline. Nitrogen has a lower thermal
conductivity than helium, but is much less expensive. In a flame ionization detector, thermal
conductivity is of no consequence; therefore, nitrogen is preferred.

[vinney12]

Wow, this really help

[blakcmamba]

Great answer, keep it coming