Question

How many oxygen atoms are present in 0.36 g of a drop of water at STP?

• A. $6.023 \times 10^{22}$
• B. $1.205 \times 10^{22}$
• C. $6.023 \times 10^{23}$
• D. $1.205 \times 10^{23}$

Hint:

Counting atoms shortcut: \begin{itemize} \item Convert mass $\rightarrow$ moles \item Multiply by Avogadro number \item Multiply by atoms per molecule \end{itemize}

Answer 1

The Correct Option is B

Concept: Use mole concept and Avogadro's number. Step 1: Moles of water Molar mass of H$_2$O = 18 g/mol \[ \text{Moles} = \frac{0.36}{18} = 0.02 \text{ mol} \] Step 2: Number of molecules \[ 0.02 \times 6.023 \times 10^{23} = 1.205 \times 10^{22} \text{ molecules} \] Step 3: Oxygen atoms Each molecule contains 1 oxygen atom. Thus number of oxygen atoms: \[ 1.205 \times 10^{22} \]