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  4. The number of molecules in 18 mg of water in terms of Avogadro number N is

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Q. The number of molecules in $18 \,mg$ of water in terms of Avogadro number $N$ is

J & K CETJ & K CET 2009Some Basic Concepts of Chemistry

Solution:

Given, weight of water $ =18\,mg=18\times 10^{-3}g $
$ \therefore $ Number of moles $ =\frac{\text{weight}}{\text{molar}\,\text{mass}} $
$ =\frac{18\times 10^{-3}}{18}=1\times 10^{-3} $
and number of molecules = no. of moles $ \times $ Avogadro number.
$ =\,1\times 10^{-3}\times N $ $ =10^{-3}N. $