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Model Paper 10 — Chemistry STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 ScienceChemistryModel Paper 101 Mark
Question
Assertion (A): Atomic mass has no unit but is expressed in amu.
Reason (R): It is the average mass of an atom taking care of the relative abundance of all its possible isotopes.
✓

Answer

(a) Both A and R are true and R is the correct explanation of A .
Explanation: Atomic mass $=\frac{\text { Average mass of an atom }}{1 / 12 \times \text { Mass of an atom of }{ }^{12} C }$
Average mass of an atom
$
=\frac{R \cdot A(1) \times MNo+R \cdot A(2) \times M \cdot No}{R \cdot A(1)+R \cdot A(2)}
$
Here R.A = Relative abundance, M.No = Mass number and 1 and 2 refers the two possible isotopes. As atomic mass is a ratio so its has no unit.

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