Question

If 1 mL of water contains 20 drops, what is the number of water molecules in the one drop of water?
($N_A=$ Avogadro number)

• A. $\frac{0.5}{18}{N}_A$
• B. $0.05N_A$
• C. $\frac{0.05}{18}{N}_A$
• D. $0.5N_A$

Answer 1

Answer: (C)

$\frac{0.05}{18}{N}_A$

Volume of $20$ drops $=$ $1ml$

$\Rightarrow$ Volume of $1$ drop= $\frac{{10}^{-3}}{20}L=\frac{1}{20}ml$

Mass of water $=\frac{1}{20}$.....[if density= $1g/c{m}^3$]

$\therefore$ Moles of water $=\frac{1}{20}\times \frac{1}{18}=\frac{0.05}{18}$

$\therefore$ Number of molecules $=\frac{0.05}{18}{N}_A$