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How many molecules are in a drop of water?

How many molecules are contained in one drop (0.050 mL) of water (density 1.0 g/mL)?

Solution This solution is FREE courtesy of BrainMass!

To answer this question one must first start with the concept of a mole.

The mole is a number 6.022 x10^23 that can represent anything. This number is known as Avogadro's number. One would like to own a mole of pennies for instance.

The mole is only convenient when speaking of particles, atoms, molecules etc. One mole of a substance weighs the molar mass of that substance. The molar mass of a substance is the sum of all the parts of the particles/atoms/molecules whichever one is talking about. In this case we are talking about water. The water molecule has the formula H2O, and we need to add up the molar mass of (2 x H) + (1 x O). From the periodic table, we find that water has a molar mass of 18.02 g/mol. The droplet of water is assumed to be 0.050 mL which equals 0.050 g (using the density of 1 g/mol). Dividing the mass by the molar mass one will calculate the number of moles present in one drop.
0.050 g/18.02 g mol^-1 = 0.00277 or 2.8 x 10^3 moles

The number of moles multiplied by Avogadro's number will now convert the number of moles into the number of molecules.

(6.022 x 10^23) * (2.8 x 10^-3) = Answer