Question

$1mL$ 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 $=1g/mL$ )

• A. $\frac{0.02}{9}{N}_0$
• B. $\frac{18}{25}{N}_0$
• C. $\frac{25}{18}{N}_0$
• D. $\frac{0.04}{25}{N}_0$

Answer 1

Answer: (A)

Hint: Volume of one drop $=\left(\frac{1}{25}\right)mL$
$\therefore \text{ Mass of }1\text{ drop }=V\times d$
$=\left(\frac{1}{25}mL\right)(1g/mL)$
$=\frac{1}{25}g$
Number of moles of $H_{2}O=\frac{\text{ Massof water in one drop }}{\text{ Molarmass of water }}=\frac{\frac{1}{25}}{18}=\frac{1}{25\times 18}$
$\therefore \text{ Number of }{H}_{2}O\text{ Molecule }=\frac{1}{25\times 18}{N}_0$
$=\frac{1}{50\times 9}{N}_0=\frac{0.02}{9}{N}_0$