Question

Identify the pair of gases which have same number of molecules at S.T.P?

• A. 11 g of $CO_{2}$ and 14 g of $N_{2}$
• B. 16 g of $O_{3}$ and 16 g of $CH_{4}$
• C. 5 g of $H_{2}$ and 40 g of $CH_{4}$
• D. 28 g of $N_{2}$ and 22 g of $CO_{2}$

Hint:

Same Moles = Same Molecules. Just divide the given mass by the molar mass!

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
According to Avogadro's law, equal volumes of gases at STP contain the same number of molecules. This means they must have the same number of moles ($n = \text{mass} / \text{molar mass}$).
Step 2: Key Formula or Approach:
Calculate moles for each pair: $n = w/M$.
Step 3: Detailed Explanation:

Option 1: $CO_2 (11/44 = 0.25)$, $N_2 (14/28 = 0.5)$. No.
Option 2: $O_2 (16/32 = 0.5)$, $CH_4 (16/16 = 1.0)$. No.
Option 3: $H_2 (5/2 = 2.5)$, $CH_4 (40/16 = 2.5)$. Yes.
Option 4: $N_2 (28/28 = 1)$, $CO_2 (22/44 = 0.5)$. No.

Step 4: Final Answer:
The pair with the same number of molecules is 5 g of H₂ and 40 g of CH₄.