Assertion :Molar mass of $C a$ is not always $40$ gm. Reason: Average molar mass of $C a$ is $40$ gm and is calculated by abundance percentage of all isoptopes of $C a$ .

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is correct but Reason is incorrect

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Both Assertion and Reason are incorrect

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Solution

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope).

Average atomic mass $= f 1 M 1 + f 2 M 2 + . . . + f n M n$ where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.
Therefore, Molar mass of $C a$ is not always 40 gm because of presence of isotopes.

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