Male Speaker: Okay, now let's bring this back to the rule of chemistry okay. Let's say that we have 4.673 times ten to the eleven atoms of zinc and the question is how many moles do we have. Okay, now listen, look at 4.673 times ten to the eleven atoms is a lot of atoms. Is it mole of atoms, no its not because it not times ten to the 23 and if it was we will have around a whole mole of zinc, we don't, we have less than that but how many moles do we actually have, less than one.
What's the calculation, we have that many atoms of zinc but we want to multiply it by a ratio where the atoms of zinc that we just talked about here will cancel out and what are we going to left is moles, moles of what unless you abbreviation, moles of zinc, what's the ratio? Every time we have one mole of zinc, you got 6.02 times ten to the 23 atoms, that's so small but what atoms of zinc cancel and you are left with moles of zinc, so what you would effect end of doing is taking this number and dividing by 6.02 times ten to the 23, what do you get, you get 7.7625 times ten to the negative 13 moles of zinc.
Now of course that number is good enough because we need to have that same number that everybody else no -- would have if you are doing this calculation. Everybody ends in the world, do you understand? We can do that as well. Now look if these four significant digits here which there are and three here, how many we will have to keep, three that's right. This number has 1, 2, 3, 4,5 right now or six and that's not good enough, we have got six five there, we got to keep get it to three significant digits, so 7.76 stop there, got number right there is a two, we over round that up and round it down and so your answer is 7.76 times ten to the negative 13 moles of zinc. Is that less than one mole, ten to the negative 13 is a number less than one, its not a negative number, that's a number that's less than one, greater than zero, it supposed to be because this number is less than one mole so it makes sense that's good.
So, we will be converting atoms or molecules to moles and moles to molecules now. How about introducing the concept of the mass of an atom and how many moles in that takes up or how many grams are there in a mole, okay. So, on the periodic table you will notice on the top right hand corner of every element's little box that it occupies you got a number there which actually is an average mass of all the isotopes of that element on planet. There the number of isotopes and their percentages of how much they are found taken into account to make up something called the molar mass.
Now, for hydrogen on the periodic table molar mass is 1.01, that means its 1.01 grams of hydrogen that's h, when you have got that you have got one mole of h, now what's the molar mass of water, while water is H2O. An H2O has two Hs in it at 1.01 grams per mole, that's how many grams are in one mole. Oxygen, there is only one of these oxygen is here at a molar mass of 16.00 grams for one mole. One mole of oxygen weighs more than one mole of hydrogen, well yeah because oxygen has more protons, neutrons and electrons in one atom, then hydrogen does.
So, if you have 6.02 times ten to the 23 atoms of each, oxygen's more than the hydrogen. What's the molar mass of water then? You just take two waters that 1.01 and add it to 16 on in your calculator, of course you have 18.02 grams of water every time you have one mole or 6.02 times ten to the 23 molecules of water.
So, it's really easy. You just look at the formula, so if I wrote a formula here and the formula that was complicated is P4O10 tetra phosphorous deca oxide, takes four molar masses of phosphorous on your chart, 30.97 and then add that to 10 16s, you can just punch that into your calculator, add four times 30.97 plus, because your calculator doesn't work for operations, ten times 16 you got it and most periodic tables, not all but most have two numbers after the decimal for every molar mass and is just like adding a whole bunch masses together and when you add or subtract you retain the least number of decimal places.
So, every molar mass you calculate, if your periodic table has two numbers after the decimal, keep two numbers after the decimals. So, people look at it say -- the idea. When you are adding or subtracting columns of number which in effect were doing here you retain a least number of decimal places. So, always just keep two. Now let's have some fun.
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