Hey guys, let's do another example about electric charge, okay? Water weighs one kilogram per liter has a molecular weight of 18 grams per mole and has 10 electrons per molecule.
Part A how many electrons does two liters of water have? An Part B, what charges, what charge do these electrons represent? So, how many electrons do we find in Part A? what is the charge of those electrons, okay? So, for Part A. First, what we want to do is we want to figure out how to get from liters which is what we're given, we're given two liters of water to number of electrons, this will tell us how to solve the problem, we need to create a sort of map to the solution, okay? Let's start with liters, right? Because that's what's given to us, what can we go to next? Well, we're told that there's a conversion between kilograms and liters that we can say for every liter of water it has a mass of one kilogram. So, we know how to go from liters to kilograms, next we have grams to moles. Now, we don't have kilograms to moles but we know right away that one kilogram is 1,000 grams. So, we can easily go from kilograms to grams and then using the conversion go from grams to moles. Now, our last conversion is electrons per molecule, we don't have our number of molecules yet, we have in moles but we can use Avogadro's number to convert moles to molecules. Now, using our last conversion factor, we can go from molecules to number of electrons. So, this right here is our map, that's going to guide us through this problem, okay? So, let's start doing these conversions, 2 liters of water times 1 kilogram per liter is 2 kilograms. So, our water has a mass of 2 kilograms now right away, we know that that's equivalent to 2,000 grams, okay? So, we've done this step and this step. Now, we need to go from grams to moles, okay? 2,000 grams times 18 grams per one mole is about 111 moles. So, we've done the next step. Now, we need to go from moles to molecules using Avogadro's number 111 moles times 1 mole per 6 times 10 to the 23 molecules is 6. Seven times 10 to the 25 molecules of water, okay? So, we've done this step, the last step is simply to figure out how many electrons are represented by this much water as many molecules of water, we know that it's 10 electrons per molecule. So, it's very simple, we just multiply this number by 10, 6.7 times 10 to the 26 electrons, okay? And we followed our map successfully from liters which was given to us to electrons, okay? Now, part be, what charge does this amount of electrons represent? Well, each electron has a charge e, the elementary charge and we have some number of electrons in which we figured out in part A. So, multiplying these together will tell us our total charge our number is 6.7 times 10 to the 26 and the elementary charge is what? Remember, guys you need to know this 1.6 times 10 to the negative 19 coulombs multiplying those together we get a total charge of 1.07 times 10 to the eighth, cool? Okay.
Practice: How many electrons do you have to add to decrease the charge of an object by 16uC?