Question

An organic compound made of C, H and N contains 20% nitrogen. Its molecular weight will be:

• A. $65$
• B. $70$
• C. $100$
• D. $140$

Answer 1

The correct option is B

$70$

Explanation for correct option:

B. $70$

Step 1: Minimum molecular weight:

• “The minimum molecular weight will be the total mass of element whose percentage is given.”
• It can be expressed as:
• $\mathrm{Minimum}\mathrm{molecular}\mathrm{weight}=\frac{100}{\mathrm{percentage}\mathrm{of}\mathrm{element}\mathrm{given}}\times \mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{same}\mathrm{element}$

Step 2: Given Information:

• Percentage of nitrogen = $20\%$

Step 3: Calculation for molecular weight:

• Let's consider the total molecular weight to be ‘x’, so 20 % of x will give me the atomic weight of Nitrogen. Therefore, 20% of x = atomic weight of nitrogen.
• On solving;

$$\begin{gathered} 20\%\times \mathrm{x}=14 \\ \frac{20}{100}\times \mathrm{x}=14 \\ \mathrm{x}=\frac{14\times 100}{20} \\ \mathrm{x}=70 \end{gathered}$$

• Thus, the molecular weight of the compound will be $70$.

Explanation for incorrect options:

The molecular weight of the compound will be $70$. Thus, options A, C, and D are incorrect.

Hence, the correct option is B i.e. $70$.