Question

Calculate the number of molecules in a drop of water weighing $0.07$ g.

• A. $2.34\times {10}^{25}$ molecules of $H_{2}O$
• B. $8.6\times {10}^{24}$ molecules of $H_{2}O$
• C. $2.34\times {10}^{23}$ molecules of $H_{2}O$
• D. $2.34\times {10}^{21}$ molecules of $H_{2}O$

Answer 1

Answer: (D)

$2.34\times {10}^{21}$ molecules of $H_{2}O$

$0.07g$ of water corresponds to $\frac{0.07g}{18g/mol}=0.0039mol$.
It corresponds to $0.0039\times 6.023\times {10}^{23}=2.34\times {10}^{21}$ molecules.
Thus, the number of molecules in a drop of water weighing $0.07g$ is $2.34\times {10}^{21}$.