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MDCAT
Chemistry
2023

Chemistry · Stoichiometry

Work through this past-paper style MCQ, then read the full explanation. Practice more chemistry questions on mMCQ with adaptive practice and topic analytics.

Question

The same moles of H2, N2, and O2 are present in 0.1 cc volume at STP. Which one has the greatest number of molecules:

Options
  • A

    N2

  • B

    H2

  • C

    O2

  • D

    Number of molecules are equal

Explanation

The number of molecules is given by the formula:


Number of molceules = moles x avogadros constant

Since all of the given elements have the same number of moles all of them will have the same number of molecules, which will be 6.02 x 1023, the Avogadros constant.

This option is incorrect because the number of molecules depends only on the number of moles, not on the type of gas.

Avogadro’s law states that equal moles of gases contain equal numbers of molecules, regardless of their nature.

Since all gases here (H₂, N₂, and O₂) have the same number of moles, they must contain the same number of molecules.

Molecular weight or gas type does not affect molecule count in this case.

Thus, N₂ doesn’t have more molecules than others.

Hence, this option is wrong.

This option is also incorrect for the same reason.

Even though H₂ is lighter, that does not change the number of molecules per mole.

At STP, one mole of any gas occupies 22.4.

If all three gases have equal moles, they have equal numbers of molecules.

The molecular size or mass does not affect the molecule count per mole.

Therefore, H₂ will not have more molecules than N₂ or O₂.

So, this option is also incorrect.

The number of molecules is given by the formula:


Number of molceules = moles x avogadros constant

Since all of the given elements have the same number of moles all of them will have the same number of molecules, which will be 6.02 x 1023, the Avogadros constant.

This option is correct.

By Avogadro’s Law, equal moles of all gases contain equal numbers of molecules under the same conditions of temperature and pressure.

Here, H₂, N₂, and O₂ are all present in equal moles, so each will have the same number of molecules.

Regardless of their atomic masses or molecular sizes, the molecule count remains identical.

Hence, there is no difference in the number of molecules among them.

Therefore, the number of molecules are equal.

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Tagged under Chemistry · Stoichiometry · 2023