Question

$1$ mL of water has $25$ drops. Let $N_A$ be the Avogadro number. What is the number of molecules present in $1$ drop of water? (Density of water = $1$ g/mL)

• A. 0.02/9 No
• B. 18/25No
• C. 25/18 No
• D. 0.04/25 No

Answer 1

The Correct Option is A

We are given that: - 1 mL of water contains 25 drops - The density of water = 1 g/mL - The molecular weight of water (H₂O) = 18 g/mol - $N_A$ is Avogadro's number

Step 1: Calculate the mass of 1 drop of water. The mass of 1 mL of water = 1 g, and there are 25 drops in 1 mL, so the mass of 1 drop of water is: 
Mass of 1 drop = $\dfrac{1}{25}$ g

 Step 2: Calculate the number of moles in 1 drop of water. The number of moles of water in 1 drop is: 
$\text{Moles of water} = \dfrac{\text{Mass of 1 drop}}{\text{Molar mass of water}} = \dfrac{1/25}{18}$ mol 

Step 3: Calculate the number of molecules in 1 drop of water. The number of molecules in 1 drop is: 
$\text{Number of molecules} = \text{Moles of water} \times N_A = \dfrac{1}{25} \times \dfrac{1}{18} \times N_A$ This simplifies to: 
$\text{Number of molecules} = \dfrac{1}{25 \times 18} \times N_A = \dfrac{1}{450} \times N_A$ Finally, we get: 
Answer: $\dfrac{0.02}{9} \times N_A$