Question

Write the molecular formula for a compound with the possible elements C, H, N and O that exhibits a molecular ion at M⁺ =128.063.

Exact Masses of the Most Abundant Isotope of Selected Elements

Isotope	Natural abundance (%)	Exact mass
1H	99.985	1.008
12C	98.90	12.000
14N	99.63	14.003
16O	99.76	15.995

Molecular formula  (In the order CHNO, with no subscripts)

Answer 1

Here you can use rule of thirteen for determination of number of carbon and hydrogen . Once you determine that you can estimate molecular formula based on presence of heteroatoms. So let's see rule of dividing

(128 /13) = 9 and 11 remain as arrears, so number of carbon and hydrogen =

C₉H₉ + 11 (arrears )

Now by awarding 11 arrears to same    number of hydrogen ,so

C₉H ₂₀

But as per input, there is one nitrogen and oxygen present, so you can manage number of carbon and hydrogen as per correction needed for introduction of heteroatoms .Here

One Oxygen equivalent to = CH₄

Because molar mass of carbon = 12.00g/mol , molar mass for four hydrogen = 1.008 x 4 =4.032 , so one oxygen correspond to one carbon and four hydrogen ,hence molecular formula after subtracting CH₄ ,

C₈H₁₆O

but still we have to introduce one nitrogen atom , correction for one nitrogen equivalent to CH₂ , here we can subtract CH₂ from above molecular formula ,then final molecular formula becomes -

C₇H₁₄ON final Formula.