Question

Acetylene and butene have empirical formula $\text{CH}$ and $\text{C}{\text{H}}_{\text{2}}$ respectively. The molecular mass of acetylene and butene are $\text{26g}$ and $\text{56g}$ respectively. Deduce their molecular formula.
(A)- ${\text{C}}_{\text{2}}{\text{H}}_{\text{2}}$ and ${\text{C}}_{\text{2}}{\text{H}}_{\text{2}}$
(B)- ${\text{C}}_4{\text{H}}_8$ and ${\text{C}}_{\text{2}}{\text{H}}_{\text{2}}$
(C)- ${\text{C}}_{\text{2}}{\text{H}}_{\text{2}}$ and ${\text{C}}_4{\text{H}}_8$
(D)- ${\text{C}}_{\text{2}}{\text{H}}_4$ and ${\text{C}}_4{\text{H}}_{10}$

Answer 1

Hint: Acetylene belongs from the alkyne family where between 2 carbon atoms at least one triple bond is present and butene belongs from the alkene family where between 2 carbon atoms at least one double bond is present.

Complete step by step solution: Given that, molecular mass of acetylene = $\text{26g}$,
And molecular mass of butene = $\text{56g}$.
-General chemical formula for the representation of alkene is ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n}}$.
-General chemical formula for the representation of alkyne is ${\text{C}}_{\text{n}}{\text{H}}_{\text{2n + 2}}$.
(Where, n is the no. of carbon atom in the molecule.)
-In acetylene 2 carbon atoms are present & between these 2 carbons triple bond is present. And according to the general chemical formula of alkynes, chemical formula of acetylene is ${\text{C}}_{\text{2}}{\text{H}}_{\text{2}}$ and with bonds it is shown as $\text{CH}\equiv \text{CH}$.
-In butene total 4 carbon atoms are present & between first, 2 carbon atoms double bond is present. According to the general chemical formula of alkenes, chemical formula of butene is ${\text{C}}_4{\text{H}}_8$ and with bonds it is shown as $\text{C}{\text{H}}_{\text{2}}\text{ = CH - C}{\text{H}}_{\text{2}}\text{ - C}{\text{H}}_{\text{3}}$.
-From the chemical formula of Acetylene and butene, it is confirmed that the molecular mass of Acetylene is $\text{26g}$ & butene is $\text{56g}$.

Hence, option (C) is correct.

Note: In this question some of you may get confused in the presence of no. of double bonds in the butene molecule. So, always keep in mind that until the numbering of carbon atoms is not given, we will assume that only one double bond is present.