Among the following, choose the ones with equal number of atoms. A. 212 g of Na2CO3 (s) [molar mass = 106 g] B. 248 g of Na2O (s) [molar mass = 62 g] C. 240 g of NaOH (s) [molar mass = 40 g] D. 12 g of H2 (g) [molar mass = 2 g] E. 220 g of CO2 (g) [molar mass = 44 g] Choose the correct answer from the options given below :

The correct answer is Option 2. Full mathematical solution steps and hints are accessible in the interactive StemCET workspace.

Chemistry Q68 – NEET-UG 2025

Among the following, choose the ones with equal number of atoms. A.

212 g of {Na}_{2} {CO}_{3} (s) [molar mass = 106 g]

248 g of {Na}_{2} O (s) [molar mass = 62 g]

240 g of NaOH (s) [molar mass = 40 g]

12 g of H_{2} (g) [molar mass = 2 g]

220 g of {CO}_{2} (g) [molar mass = 44 g]

Choose the correct answer from the options given below :

The number of atoms in a given mass of a substance depends on the number of moles of the substance and the number of atoms per molecule/formula unit.

● Video Walkthrough

Step 1: Calculate Moles for Each Substance✦ Active

For each given substance, calculate the number of moles ( $n$ ) using the formula $n = \frac{given mass}{molar mass}$ :

A: ${Na}_{2} {CO}_{3}$ : $n_{A} = \frac{212 g}{106 g/mol} = 2 mol$

B: ${Na}_{2} O$ : $n_{B} = \frac{248 g}{62 g/mol} = 4 mol$

C: $NaOH$ : $n_{C} = \frac{240 g}{40 g/mol} = 6 mol$

D: $H_{2}$ : $n_{D} = \frac{12 g}{2 g/mol} = 6 mol$

E: ${CO}_{2}$ : $n_{E} = \frac{220 g}{44 g/mol} = 5 mol$

Step 2: Determine Atoms per Molecule/Formula Unit○ Expand

Count the total number of atoms in one molecule or formula unit for each substance:

A: ${Na}_{2} {CO}_{3}$ has $2 (Na) + 1 (C) + 3 (O) = 6$ atoms.

B: ${Na}_{2} O$ has $2 (Na) + 1 (O) = 3$ atoms.

C: $NaOH$ has $1 (Na) + 1 (O) + 1 (H) = 3$ atoms.

D: $H_{2}$ has $2 (H) = 2$ atoms.

E: ${CO}_{2}$ has $1 (C) + 2 (O) = 3$ atoms.

Step 3: Calculate Total Number of Atoms and Compare○ Expand

Multiply the number of moles by the number of atoms per molecule/formula unit and Avogadro's number ( $N_{A}$ ) to get the total number of atoms:

A: Total atoms = $2 mol \times 6 atoms/molecule \times N_{A} = 12 N_{A}$

B: Total atoms = $4 mol \times 3 atoms/molecule \times N_{A} = 12 N_{A}$

C: Total atoms = $6 mol \times 3 atoms/molecule \times N_{A} = 18 N_{A}$

D: Total atoms = $6 mol \times 2 atoms/molecule \times N_{A} = 12 N_{A}$

E: Total atoms = $5 mol \times 3 atoms/molecule \times N_{A} = 15 N_{A}$

Comparing the results, options A, B, and D all have $12 N_{A}$ atoms. Therefore, they have an equal number of atoms.

💡 Teacher's Secret Hint

Remember that Avogadro's number ( $N_{A}$ ) is a constant, so we only need to compare the product of moles and atoms per molecule.

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