The number of water molecules present in a drop of water at room temperature

Water drops come in different sizes.
Let's imagine a drop weighs a quarter of a gram.
The molar mass of water is about 18g/mol, which means that 6.02 x 10^23 water molecules (AKA a mole of water molecules) weigh about 18 grams.
A quarter of a gram is 1/72 of 18, so it contains 1/72 times 6.02 x 10^23 molecules. That equals 8361111111111110000000 molecules.
In scientific notation that is... 8.36 x 10^21 molecules.

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The answer is c) pyrite