Calculate the no. of molecules in a drop of water weighing `0.07 g`.

Explanation

To calculate the number of molecules in a drop of water weighing 0.07 g, we can follow these steps:

### Step 1: Determine the molar mass of water (H₂O)
The molar mass of water (H₂O) can be calculated as follows:
- Hydrogen (H) has an atomic mass of approximately 1 g/mol. Since there are 2 hydrogen atoms in water, the total mass from hydrogen is:
\[ 2 \times 1 \, \text{g/mol} = 2 \, \text{g/mol} \]
- Oxygen (O) has an atomic mass of approximately 16 g/mol. Therefore, the total molar mass of water is:
\[ 2 \, \text{g/mol} + 16 \, \text{g/mol} = 18 \, \text{g/mol} \]

### Step 2: Calculate the number of moles in 0.07 g of water
Using the formula for moles:
\[ \text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} \]
Substituting the values:
\[ \text{moles} = \frac{0.07 \, \text{g}}{18 \, \text{g/mol}} \approx 0.00389 \, \text{moles} \]

### Step 3: Calculate the number of molecules
To find the number of molecules, we use Avogadro's number, which is approximately \(6.02 \times 10^{23}\) molecules/mol. The formula is:
\[ \text{number of molecules} = \text{moles} \times \text{Avogadro's number} \]
Substituting the values:
\[ \text{number of molecules} = 0.00389 \, \text{moles} \times 6.02 \times 10^{23} \, \text{molecules/mol} \approx 2.34 \times 10^{21} \, \text{molecules} \]

### Final Answer
The number of molecules in a drop of water weighing 0.07 g is approximately:
\[ 2.34 \times 10^{21} \, \text{molecules} \]

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