Question

Where would an unknown with a retention index of 936 be eluted in Figure 23-7?

Short answer

The eluted unknown with a retention index of 936 would be found after nonane.

Step-by-step solution

Step 1: Concept used

Kovats retention index:

For linear alkanes, the retention index, I , equals 100times the number of carbon atoms.

For a molecule of Octane, I=800 calculated using the formula

$\mathbf{I}\mathbf{=}\mathbf{100}\left[n+\left(N-n\right)\frac{{\mathrm{logL}}^{\text{'}}\left(\left(\mathrm{unknown}\right)-\mathrm{logL}\text{'}\left(n\right)\right)}{{\log }_4\left(N\right)-{\log }_{4\text{'}}\left(n\right)}\right]$

where

n=number of carbon atoms in smaller alkane

N=number of carbon atoms in larger alkane

${{\mathbf{t}}_{\mathbf{r}}}^{\mathbf{\text{'}}}\left(n\right)\mathbf{=}$adjusted retention time of smaller alkane

localid="1663581965257" ${\mathbf{t}}_{\mathbf{r}}\mathbf{\text{'}}\mathbf{\left(}\mathbf{N}\mathbf{\right)}\mathbf{=}$adjusted retention time of larger alkane.

Position of the unknown

Nonane has a retention value of 900, whereas the unknown has a retention index of 936. Therefore, the eluted unknown with a retention index of 936 would be found after nonane.