There are 1.67 sextillion H2O molecules in a drop of water. This is determined by using the formula of 50 microliters per drop of water.
Anyone can figure out the number of atoms in any amount of water by simply using a formula that involves millimoles and knowing that each mole in a compound has 6.022 x 10 to the power of 23. The microliters and milimoles can then be converted into domestic terms to find out exactly how many molecules are in any amount of water.
This method is complicated and useless to most people because most people don't need to know the amount of molecules in larger amounts of water. However, knowing the number of H2O molecules in a drop of water does make figuring out how many are in larger amounts easier. A person can measure the amount of water they are wishing to find the molecular number of and then multiply the number of drops by 1.67 sextrillion to get the exact number of H2O molecules that are present.
It is important for scientists, as well as meteorologists and oceanologists, to understand H2O and the amount of molecules that are in the specific amount of water they are studying. This number helps them to better determine how the water will react to different situations.