Calculate the number of molecules in a drop of water weighing 0.09 g.

Answer

To calculate the number of molecules in a drop of water weighing 0.09 g, we can follow these steps:

### Step 1: Determine the molecular weight of water (H₂O)
Water (H₂O) consists of two hydrogen atoms and one oxygen atom. The atomic weights are approximately:
- Hydrogen (H): 1 g/mol
- Oxygen (O): 16 g/mol

The molecular weight of water can be calculated as follows:
\[ \text{Molecular weight of H₂O} = (2 \times 1) + (1 \times 16) = 2 + 16 = 18 \text{ g/mol} \]

### Step 2: Calculate the number of moles of water in 0.09 g
Using the formula for calculating moles:
\[ \text{Number of moles} = \frac{\text{mass (g)}}{\text{molecular weight (g/mol)}} \]
Substituting the values:
\[ \text{Number of moles} = \frac{0.09 \text{ g}}{18 \text{ g/mol}} = 0.005 \text{ moles} \]

### Step 3: Calculate the number of molecules using Avogadro's number
Avogadro's number is approximately \(6.022 \times 10^{23}\) molecules/mol. To find the number of molecules, we use the formula:
\[ \text{Number of molecules} = \text{Number of moles} \times \text{Avogadro's number} \]
Substituting the values:
\[ \text{Number of molecules} = 0.005 \text{ moles} \times 6.022 \times 10^{23} \text{ molecules/mol} \]
Calculating this gives:
\[ \text{Number of molecules} = 3.011 \times 10^{21} \text{ molecules} \]

### Final Answer
The number of molecules in a drop of water weighing 0.09 g is approximately \(3.011 \times 10^{21}\) molecules.

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