Introduction:
In ordinary water, there is a small amount of heavy water present. We need to determine the number of heavy water molecules in a drop of water with a volume of 0.01 ml.

Given:
- Ordinary water contains 1 part of heavy water per 6000 parts by weight.
- Volume of water drop = 0.01 ml.

Calculating the number of heavy water molecules:
To calculate the number of heavy water molecules in a drop of water, we need to follow these steps:

Step 1: Calculate the weight of water in the drop.
- Density of water is approximately 1 g/ml.
- Volume of the drop = 0.01 ml.
- Weight of water in the drop = Volume × Density = 0.01 g.

Step 2: Calculate the weight of heavy water in the drop.
- In ordinary water, there is 1 part of heavy water per 6000 parts by weight.
- Weight of heavy water in the drop = (Weight of water in the drop) / 6000 = 0.01 / 6000 g.

Step 3: Calculate the number of heavy water molecules.
- 1 mole of any substance contains 6.022 × 10^23 molecules (Avogadro's number).
- The molar mass of heavy water (D2O) is approximately 20 g/mol.
- Number of moles of heavy water = (Weight of heavy water in the drop) / (Molar mass of heavy water) = (0.01 / 6000) / 20 mol.
- Number of heavy water molecules = (Number of moles of heavy water) × (Avogadro's number) = (0.01 / 6000) / 20 × 6.022 × 10^23.

Result:
The number of heavy water molecules present in a drop of water with a volume of 0.01 ml is approximately equal to (0.01 / 6000) / 20 × 6.022 × 10^23.