Question

\( x \) mg of an organic compound was analysed by Kjeldahl method. The ammonia evolved was absorbed in 50 mL of 0.5 M \( H_2SO_4 \). The unused acid required 60 mL of 0.5 M \( NaOH \) solution for complete neutralisation. If the percentage of nitrogen in the compound is \( 56 \), the value of \( x \) is:

• A. \( 500 \)
• B. \( 250 \)
• C. \( 750 \)
• D. \( 375 \)

Hint:

For Kjeldahl method calculations, use: \[ \text{Moles of acid used} = \text{Total acid} - \text{Unused acid} \] to determine nitrogen content.

Answer 1

The Correct Option is C

Step 1: Understanding the Kjeldahl Method The Kjeldahl method determines nitrogen content by: 1. Digesting the organic compound. 2. Converting nitrogen into ammonia. 3. Absorbing ammonia in sulphuric acid. 4. Neutralizing unused acid using sodium hydroxide. Step 2: Finding Unused Acid Total acid used: \[ \text{Moles of } H_2SO_4 = 50 \times 0.5 \times 10^{-3} = 0.025 \text{ moles} \] Unused acid: \[ \text{Moles of } NaOH = 60 \times 0.5 \times 10^{-3} = 0.03 \text{ moles} \] Since 1 mole of \( H_2SO_4 \) neutralizes 2 moles of \( NaOH \): \[ \text{Moles of unused } H_2SO_4 = \frac{0.03}{2} = 0.015 \text{ moles} \] Thus, used acid: \[ 0.025 - 0.015 = 0.01 \text{ moles} \] Step 3: Calculating Nitrogen Nitrogen from ammonia neutralization: \[ \text{Mass of nitrogen} = 0.01 \times 14 \times 10^3 = 140 \text{ mg} \] Step 4: Finding \( x \) \[ \frac{140}{x} \times 100 = 56 \] \[ x = \frac{140 \times 100}{56} = 250 \text{ mg} \] Conclusion Thus, the correct answer is: \[ 750 \text{ mg} \]