Question

Molecular mass of butane is:
(A)- 54
(B)- 56
(C)- 58
(D)- 60

Answer 1

Hint: For calculating the molecular mass of any species always add atomic masses of the atoms which are present in that molecule, because the molecule is formed by the combination of atoms only.

Complete step by step solution: To calculate the molecular mass of butane we have to add all atomic masses of carbon atoms and hydrogen atoms present in the molecular formula of butane.
-Butane is an alkane whose molecular formula will be written on the basis of the general formula of alkane ${C_n}{H_{2n + 2}}$, so butane is written as ${{\text{C}}_{\text{4}}}{{\text{H}}_{{\text{10}}}}$.
-From ${{\text{C}}_{\text{4}}}{{\text{H}}_{{\text{10}}}}$it is clear that in butane 4 carbon atoms and 10 hydrogen atoms are present.
-Atomic mass of carbon is 12.
-Atomic mass of hydrogen is 1.
-For calculating molecular mass we have to add atomic mass of 4 carbon & 10 hydrogen.
-So, molecular mass of butane is calculated as follow:
${{\text{C}}_{\text{4}}}{{\text{H}}_{{\text{10}}}}$ = $\left( {4 \times 12} \right) + \left( {10 \times 1} \right)$
${{\text{C}}_{\text{4}}}{{\text{H}}_{{\text{10}}}}$ = $48 + 10$
${{\text{C}}_{\text{4}}}{{\text{H}}_{{\text{10}}}} = 58$
i.e. molecular mass of butane is equal to 58.

Hence, option (C) is correct.

Additional information: In the structure of butane only single bonds are present because they belong from the category of alkane. It is present in the form of gas in the normal conditions i.e. at atmospheric pressure and at room temperature. It is highly flammable gas as its boiling point is very low which is equal to ${{\text{1}}^{\text{0}}}{\text{C}}$.

Note: Here, some of you may do wrong calculations if you don't have knowledge about the exact general formula of the alkane; because alkanes, alkene and alkyne sounds similar in a general way but have different general formulas. Alkenes consist of a double bond and alkynes have a triple bond.