text: Given:\n         20 Drops = 1 ml of water\n          1 Drop = How many atoms?\n\nProblem is quite simple. It just need little thought and time. \n\nIn one molecule of water there is 1 atom of oxygen and 2 atom of hydrogen. Saying that lets calculate the number of molecules in 1 ml of water then, approximating drops of equal size, we can divide it by 20 to get the molecules in 1 drop of water.\n\nI am asuming that you must know density of water (1g/ml) meaning that 1 ml of water = 1 g of water. Also you must be aware of what avogadro's number mean. Avogadro's number (6.022 x 10^23) is number of molecules in one mol of substance.\n\nSo calculate mol of water from 1 g of water.\n\n1 mol of water = 18 g of water \n       ??             =  1 g of water\n\n1 g of water x 1 mol of water/ 18 g = .0556 mol of water\n\n.0556 mol of water x (6.022 x 10^23 molecules of water / 1 mol) = 3.35 x 10^22 molecules of water in 1 g.\n[by canceling mol of water you end up with molecules of water in 1g]\n\nSo in 1 ml of water there are 3.35 x 10^22 molecules of water. In 1 drop of water there would be 3.35 x 10^22/20 = 1.67 x 10^21 molecules of water in 1 drop.\n\nBut, in 1 molecule of water there are three atoms (1 atom of O and 2 atoms of H). So divide 1.67 x 10^21/3 = 5.58 x 10^20 which are the atoms of oxygen. And if you multiply that by two you get atoms of H since there are 2 H atoms for every O atom. So number of H atom would be 1.12 x 10 ^21 atoms of H