Q111E - Chemistry (OpenStax)

Question

A compound with a molar mass of about \({\rm{42\;g/mol}}\) contains \({\rm{85}}{\rm{.7\% }}\)carbon and \({\rm{14}}{\rm{.3\% }}\) hydrogen. What is its molecular structure?

Step-by-Step Solution

Answer

The molecular structure of \({\rm{FN}}{{\rm{O}}_{\rm{2}}}\)molecule is trigonal planar as shown in the below resonating structures:

Step 1: Definition of Concept

Molecular structure gives the location of the atoms. This can be differentiated by naming the geometry that includes all electron pairs. The molecular structure includes the placement of the atoms in the molecule.

Step 2: Find it’s molecular structure

Considering the given information:

 

Mass is \({\rm{42g/mol}}\)

 

Carbon is \({\rm{85}}{\rm{.7\% }}\)

 

Hydrogen is \({\rm{14}}{\rm{.3\% }}\)

 

Now we must calculate the mole of C and H, which we will do by dividing their mass by their molecular weight, as follows:-

 

Moles of Carbon= \(\frac{{{\rm{85}}{\rm{.7\;g}}}}{{{\rm{12}}{\rm{.011\;g\;\;mo}}{{\rm{l}}^{{\rm{ - 1}}}}}}{\rm{ = 7}}{\rm{.14\;mol C}}\)

 

Moles of Hydrogen=\(\frac{{{\rm{14}}{\rm{.3g}}}}{{{\rm{1}}{\rm{.0079g mo}}{{\rm{l}}^{{\rm{ - 1}}}}}}{\rm{ = 14}}{\rm{.19 mol H}}\)

 

To calculate the formula, divide the mole by the smallest mole:-

 

\(\begin{aligned} \frac{{{\rm{7}}{\rm{.14\;mol}}}}{{{\rm{7}}{\rm{.14\;mol}}}}{\rm{ }} &=1C\\\frac{{{\rm{14}}{\rm{.19\;mol}}}}{{{\rm{7}}{\rm{.14\;mol}}}}{\rm{  }} &=2H\end{aligned}\)

 

Therefore, the formula is \({\rm{C}}{{\rm{H}}_{\rm{2}}}\), with a molecular mass of 14. However, the molecular mass of the compound is 42. As a result, the correct formula should be \({\rm{3 \times C}}{{\rm{H}}_{\rm{2}}}\) or \({{\rm{C}}_{\rm{3}}}{{\rm{H}}_{\rm{6}}}\).

 

The Lewis structure is as follows:-