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  4. Assertion: One mole of a substance always contain the same number of entities, no matter what the substance may be Reason: One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g of the 12C isotope.

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Q. Assertion: One mole of a substance always contain the same number of entities, no matter what the substance may be
Reason: One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly $12\, g$ of the $^{12}C$ isotope.

Some Basic Concepts of Chemistry

Solution:

One mole of a substance always contains the same number of entities, no matter what the substance may be which is $6.022 \times 10^{23}$. This is also the number of atoms present in $12\, g$ of ${ }_{12} C$ isotope.
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion