Given below are two statements : Statement I : One mole of propyne reacts with excess of sodium to liberate half a mole of H2 gas. Statement II : Four g of propyne reacts with NaNH2 to liberate NH3 gas which occupies 224 mL at STP. In the light of the above statements, choose the most appropriate answer from the options given below: (1) Statement I is correct but Statement II is incorrect. (2) Both Statement I and Statement II are incorrect (3) Statement I is incorrect but Statement II is correct (4) Both Statement I and Statement II are correct.

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Published July 8, 2025
Chemistry
Organic Chemistry
Alkynes
Acid-Base reactions
Stoichiometry
Gas Laws

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Detailed Explanation

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Simple Explanation (ELI5)

Imagine a Lego stick (propyne)

It has a special block (the hydrogen directly attached to the triple bond) that can be popped off quite easily when it meets certain metals or bases.

  1. When it meets shiny sodium metal (Na), two Lego sticks have to pop off one special block each to give one full hydrogen gas balloon (H2). That means one stick only makes half a balloon.
  2. When it meets a powerful Lego-popper called sodamide (NaNH2), every stick pops off one special block and sends out one puff of ammonia gas (NH3).

If you know how many sticks you have and how many puffs or balloons each reaction makes, you can count the volume of gas using the rule: 1 mole of any gas = 22.4 L at STP. That’s like saying every full balloon has the same size under standard conditions.

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Step-by-Step Solution

Step 1: Molar mass of propyne

Propyne   (CH3CCH)\;(CH_3 – C \equiv CH) has the formula C3H4C_3H_4.

M=3×12+4×1=36+4=40g mol1M = 3 \times 12 + 4 \times 1 = 36 + 4 = 40\,\text{g mol}^{-1}

Statement I

Reaction with metallic sodium:

2C3H4+2Na    2C3H3Na+H22\,C_3H_4 + 2\,Na \;\longrightarrow\; 2\,C_3H_3Na + H_2

From the equation: 2mol2\,\text{mol} propyne → 1mol1\,\text{mol} H2H_2.
Therefore 1mol1\,\text{mol} propyne → 12mol\frac{1}{2}\,\text{mol} H2H_2.
Statement I is correct.

Statement II

  1. Given mass of propyne:

    n=4g40g mol1=0.10moln = \frac{4\,\text{g}}{40\,\text{g mol}^{-1}} = 0.10\,\text{mol}

  2. Reaction with NaNH2NaNH_2:

    C3H4+NaNH2C3H3Na+NH3C_3H_4 + NaNH_2 \longrightarrow C_3H_3Na + NH_3

    Thus 1 mol propyne → 1 mol NH3NH_3.

    0.10mol propyne0.10mol NH30.10\,\text{mol propyne} \Rightarrow 0.10\,\text{mol } NH_3

  3. Volume of 0.10mol0.10\,\text{mol} NH3NH_3 at STP:

    V=n×22.4L=0.10×22.4=2.24L=2240mLV = n \times 22.4\,\text{L} = 0.10 \times 22.4 = 2.24\,\text{L} = 2240\,\text{mL}

    The statement claims 224 mL, which corresponds to only 0.01 mol NH3NH_3. Therefore Statement II is incorrect.

Final choice

Only Statement I is correct → Option (1).

Examples

Example 1

Acetylene cylinders used in welding—acetylene reacts with sodamide to generate acetylide salts in synthetic chemistry.

Example 2

Preparation of alkynyl sodium salts for nucleophilic substitution in pharmaceutical synthesis.

Example 3

Use of molar volume to quickly estimate gas quantities in laboratory reactions and titrations.

Visual Representation

References

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