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Q.
If $1\, mL$ of water contains $20$ drops then number of molecules in a drop of water is
Some Basic Concepts of Chemistry
Solution:
(i) Density of water $=1\, g / mL$.
$\therefore 1000\, mL =1000\, g$
(ii) Number of moles of water in $1000\, g (n)=w / m$
$n=\frac{1000}{18}=55.56$ moles
$\therefore$ Number of moles in $1\, mL$ of water
$(n)=\frac{55.56}{1000}$
$(n)=0.0556$ moles
(iii) $\because 1\, mL$ of $H _{2} O$ contain $20$ drops.
$\therefore$ Number of moles of $H _{2} O$ in one drop
$=\frac{0.0556}{20}$
$(n)$ per drop $=0.0027$
(iv) $\because n =\frac{N}{N_{A}}=\frac{N}{6.023 \times 10^{23}}$
$N =n \times N_{A}=0.002 \times 6.023 \times 10^{23}$
$N =0.0162 \times 10^{23}=1.62 \times 10^{21}$ molecules