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Q.
$1\, mL$ of water has $25$ drops. Let $N_0$ be the Avogadro number. What is the number of molecules present in 1 drop of water? (Density of water $=1\, g / mL$ )
Hint: Volume of one drop $=\left(\frac{1}{25}\right) m L$
$ \therefore \text { Mass of } 1 \text { drop }=V \times d$
$=\left(\frac{1}{25} m L\right)(1 g / m L)$
$=\frac{1}{25} g$
Number of moles of $H _2 O =\frac{\text { Massof water in one drop }}{\text { Molarmass of water }}=\frac{\frac{1}{25}}{18}=\frac{1}{25 \times 18}$
$\therefore \text { Number of } H _2 O \text { Molecule }=\frac{1}{25 \times 18} N_0$
$=\frac{1}{50 \times 9} N_0=\frac{0.02}{9} N_0$