The number of water molecules present in a drop of water weighing 0.018 g is

Options

A. `6.022 xx 10^(26)`
B. `6.022 xx 10^(23)`
C. `6.022 xx 10^(19)`
D. `6.022 xx 10^(20)`

Explanation

To find the number of water molecules present in a drop of water weighing 0.018 g, we can follow these steps:

### Step 1: Determine the molecular weight of water (H₂O)
The molecular weight of water (H₂O) is calculated as follows:
- Hydrogen (H) has an atomic weight of approximately 1 g/mol, and there are 2 hydrogen atoms in water.
- Oxygen (O) has an atomic weight of approximately 16 g/mol.

Thus, the molecular weight of water is:
\[ \text{Molecular weight of H₂O} = (2 \times 1) + 16 = 18 \text{ g/mol} \]

### Step 2: Calculate the number of moles of water in 0.018 g
To find the number of moles, we use the formula:
\[ \text{Number of moles} = \frac{\text{mass (g)}}{\text{molecular weight (g/mol)}} \]
Substituting the values:
\[ \text{Number of moles} = \frac{0.018 \text{ g}}{18 \text{ g/mol}} = 0.001 \text{ moles} \]

### Step 3: Calculate the number of molecules in 0.001 moles
Using Avogadro's number, which is \(6.022 \times 10^{23}\) molecules/mol, we can find the number of molecules:
\[ \text{Number of molecules} = \text{Number of moles} \times \text{Avogadro's number} \]
Substituting the values:
\[ \text{Number of molecules} = 0.001 \text{ moles} \times 6.022 \times 10^{23} \text{ molecules/mol} \]
Calculating this gives:
\[ \text{Number of molecules} = 6.022 \times 10^{20} \text{ molecules} \]

### Final Answer
The number of water molecules present in a drop of water weighing 0.018 g is \(6.022 \times 10^{20}\) molecules.

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