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NEET Chemistry · Some Basic Concepts in Chemistry

Atomic and molecular masses and the mole concept

Taught in NCERT Class 11 Chemistry, Chapter 1: Some Basic Concepts of Chemistry.

2 verified questions: Easy 0 · Medium 1 · NEET level 1

Sample questions

Sample 1Medium

How many oxygen atoms are present in 4.4 g of carbon dioxide? (C = 12 u, O = 16 u, N_A = 6.022 × 10^23 mol^-1)

  1. A

    1.204 × 10^23

  2. B

    6.022 × 10^22

  3. C

    2.409 × 10^23

  4. D

    3.011 × 10^22

Show the answer
The answer is A.

Molar mass of CO2 is 44 g mol^-1, so 4.4 g is 0.1 mol of CO2. Each molecule has two O atoms, giving 0.2 mol O atoms = 0.2 × 6.022 × 10^23 = 1.204 × 10^23 atoms.

  • B.

    This is the number of CO2 molecules (0.1 mol), not oxygen atoms.

  • C.

    This counts four O atoms per molecule, double the true number.

  • D.

    This is half the number of CO2 molecules; it neither counts molecules nor atoms correctly.

NCERT: Class 11 Chemistry, Chapter 1, 1.8

Sample 2NEET level

The number of molecules present in 4.4 g of carbon dioxide (molar mass 44 g mol⁻¹; NA = 6.022 × 10²³ mol⁻¹) is

  1. A

    6.022 × 10²³

  2. B

    6.022 × 10²²

  3. C

    1.2044 × 10²³

  4. D

    3.011 × 10²²

Show the answer
The answer is B.

Moles of CO2 = 4.4 g / 44 g mol⁻¹ = 0.1 mol. Number of molecules = 0.1 mol × 6.022 × 10²³ mol⁻¹ = 6.022 × 10²² molecules.

  • A.

    This is the number in one full mole (44 g), not 0.1 mol.

  • C.

    This is twice the correct value, as if 0.2 mol were present.

  • D.

    This is half the correct value, as if 0.05 mol were present.

NCERT: Class 11 Chemistry, Unit 1 (Some Basic Concepts of Chemistry), Mole Concept and Molar Masses

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