The mass of a water droplet is 10 to the power of -10. How many molecules does it consist of?

Units of measurement are not specified in the condition, we will consider m = 10 ^ -10 kg.
Let’s write an expression to determine the amount of a substance:
ν = m / M = N / Na, where m is the mass of the substance, M is the molar mass of the substance, N is the number of particles, Na is Avogadro’s constant Na = 6.023 * 10 ^ 23 1 / mol.
Let us express N from this expression:
N = m * Na / M
M (H2O) = 2 * 0.001 + 0.016 = 0.018 kg / mol. (using the periodic table).
N = m * Na / M = (10 ^ -10) * 6.023 * 10 ^ 23 / 0.018 = 3.34 * 10 ^ 15.
Answer: there are 3.34 * 10 ^ 15 water molecules in a drop.