Question

A big drop of water has a volume 1ml. How many molecules of water are their in this drop. If the density of water is 1g per ml.

Answer 1

For that, you would use molar calculations. A mole is the amount of a substance that contains Avogadro’s number, or 6.022*10^23 molecules. Since that number is well chosen, it’s also quite easily to calculate from molecular weights.

The basic atomic mass unit (or Dalton) is the weight of one 12th of a Carbon atom. Which is also almost exactly the weight of one proton or neutron. By looking at a periodic table, you can in fact easily count the atomic weight of one molecule of almost any substance if you have the total formula.

Water’s total formula is H20. So there’s two hydrogen atoms and one oxygen atom in a molecule of water. Since oxygen has an atomic weight of 16 and hydrogen one of 1, the atomic weight of water is 16+2*1=18.

Now. From the way avogadro’s number is defined, we know that one mole of water, that is 6.022*10^23 molecules of water, weigh exactly 18 grams. Or the other way round, if we have 18 grams of water, we know it’s 6.022*10^23 molecules. (Plus or minus our measuring precision, of course.)

Google tells me a drop of water is 0.05 grams. Good enough, I suppose.

So, we have, by basic primary school arithmetic:

6.022*10^23/18/20 molecules of water in a drop. For that, my calculator gives me:

1.67*10^21 grams.