text: the best chem problem there is!!!!!? knowing that there are 20 drops of water in a ml. how many atoms are there is a drop of water?? atoms! not molecules! given:          20 drops = 1 ml of water           1 drop = how many atoms?  problem is quite simple. it just need little thought and time.   in 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.  i 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.  so calculate mol of water from 1 g of water.  1 mol of water = 18 g of water         ??             =  1 g of water  1 g of water x 1 mol of water/ 18 g = .0556 mol of water  .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. [by canceling mol of water you end up with molecules of water in 1g]  so 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.  but, 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