Question

question 17
using the rule of 13, what is a possible formula for m=120 (100%) on a mass spectrum?
all of the above are possible molecular formulas
c8h10n
c7h8n2

Explanation:

Step1: Recall rule of 13

The rule of 13 is based on the fact that the molar - mass of a CH unit is approximately 13 g/mol. We first divide the given molar mass (M = 120) by 13.
$n=\frac{120}{13}=9$ with a remainder $r = 3$. The base formula from the rule of 13 is $C_{n}H_{n + r}$, so the base formula is $C_{9}H_{12}$. But we can also consider the effect of nitrogen. For every N atom added, we subtract 1 CH₂ unit.

Step2: Check option $C_{8}H_{10}N$

The molar - mass of $C_{8}H_{10}N$:
The molar - mass of C is approximately 12 g/mol, H is approximately 1 g/mol, and N is approximately 14 g/mol.
$M=(8\times12)+(10\times1)+14=96 + 10+14=120$ g/mol.

Step3: Check option $C_{7}H_{8}N_{2}$

The molar - mass of $C_{7}H_{8}N_{2}$:
$M=(7\times12)+(8\times1)+(2\times14)=84 + 8+28=120$ g/mol.

Answer:

all of the above are possible molecular formulas