Question

If $1mL$ of water contains $20$ drops then number of molecules in a drop of water is

• A. $6.023\times 10^{23}$ molecules
• B. $1.376\times 10^{26}$ molecules
• C. $1.62\times 10^{21}$ molecules
• D. $4.346\times 10^{20}$ molecules

Answer 1

Answer: (C)

(i) Density of water $=1g/mL$.
$\therefore 1000mL=1000g$
(ii) Number of moles of water in $1000g(n)=w/m$
$n=\frac{1000}{18}=55.56$ moles
$\therefore$ Number of moles in $1mL$ of water
$(n)=\frac{55.56}{1000}$
$(n)=0.0556$ moles
(iii) $\because 1mL$ 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