# Homework Help: Calculating Coulombs Help

1. Mar 11, 2004

### LTech221

Hi everyone its my first time here, read the forums greatly though and have been a help in understanding but now im completely lost on this problem: Calculate the number of coulombs of positive charge in 250 cm^3 of (neutral) water (about a glass full). So far I tried converting the volume to m^3 (2.5E-4 m^3, not sure if it matters.)then tried tried dividing by colulomb's constant and ended up with units kg/s^2 * c^2 and dont know where to go from there. Im not sure if I really approached this problem right as the section it supposedly comes from keeps mentioning q=ne, but I cannot relate that directly. Im thinking im missing something inbetween , any ideas to go about the problem would be appreciated.

2. Mar 11, 2004

### Philcorp

if you know the volume of the water then it should be easy enough for you to convert that into an equivalent mass. From there you should know the ratio of moles to grams for water ( i think its something like 18.02g/mol). This allows you to convert you volume from grams into moles. Now you can use Avagadro's number (6.02x10^23) to know the number of molecules in your sample. From there you know that each water molecule is made up of H_2O.....so thats 10 protons. And each proton have 1.6x10^-19 C of charge.

There you have it!

3. Mar 11, 2004

### LTech221

Ahhh, thats what i was missing, thanks for the help