Calculate the number of molecules in one drop of water.

Answer: N = 1.003 * 10 ^ 21 molecules.
Explanation of the solution to the problem: to solve this problem, you must use the following formulas:
N = Na * n, where N is the number of molecules of a given substance, Na is purely Avagadro (the value is constant and equal to: Na = 6.02 * 10 ^ 23 mol ^ -1), n ​​is the amount of substance of the given compound, mol.
The amount of substance is determined by the formula:
n = m / M, where m is the mass of the substance, g; M is the molar mass of the substance, M (H2O) = 18 g / mol (found according to the system of chemical elements of D.I. Mendeleev).
The mass of a substance is found by the formula:
m = p * v, where p is the density of the substance, ml; v is the volume of the substance, g / ml (p (H2O) = 1 g / ml).
Substituting into the formulas the values ​​known from the condition of the problem, we obtain:
m = p * v = 1 * 0.03 = 0.03 g.
N = Na * n = Na * m / M = 6.02 * 10 ^ 23 * 0.03 / 18 = 1.003 * 10 ^ 21 molecules.