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#24 - molar mass
Now, why is this important? Well, what if you had to combine aluminum(Al) with copper (II) chloride (AlCl2) without having any of the reactants left over. Well first, here�s a chemical equation: According to the chemical equation, for every 2 Al�s you need, you�ll need 3 CuCl2�s to carry out the reaction completely. You only have a scale to measure these chemicals, and this scale measures things in grams, not moles(in fact there is no such scale that measures in moles). So you�ll need to convert how many grams you�ll need to have 2 moles of Al, and how many grams you�ll need to have 3 moles of CuCl2.
Now how do you convert moles to grams? By using the atomic number. Cool. so lets use a conversion factor: _________________________________________________________________
Lets do Al first, just because so put down the information you know about Al.
Then use the atomic mass of Al as a conversion factor, it�s 26.98 (it might vary with your periodic table)
(remember: the atomic number is how many grams that�s in one mole of the element) multiply everything on the top, and multiply everything on the bottom
O.K., so lets do the same with CuCl2, but wait a minute, these are two elements with two different atomic numbers (Cu = 63.55 & Cl = 35.45) so what do you do? You would add each atomic number up for each element:
This is called the molar mass It would take 134.45 grams of the compound CuCl2 for you to have one mole This number is larger because the CuCl2 molecule is larger than the Al atom. There�s three atoms in CuCl2 compared to just one Al atom. So, if you had the same number of each, the bigger one would weigh more. Imagine if you have ten bowling balls, and ten marbles, which group of balls will be heavier, the bowling balls right? it�s like comparing Al and CuCl2; a mole of Al will weigh less than a mole of CuCl2. So lets see how many grams you�ll need for 3 moles of CuCl2:
So you would need 53.96 grams of Al, and 403.35 grams of CuCl2 Molar mass will become easier with time. In chemistry, you�ll quickly get used to doing this. | ||||||||||||||||||||||||||||||||||||||||||||||
Now do you remember what the atomic masses on the periodic table meant?
The average of the number of protons and neutrons in the nucleus of a certain atom. Well, there is another meaning to what the atomic mass is. It is also how many grams will one mole of that substance weigh. For example, the atomic number of oxygen is 16, so if you had one mole of oxygen, it would weigh 16 grams. It works both ways, the atomic number of Aluminum is 26.98, so if you had 26.98 grams of Aluminum in your hand, you�d be holding one mole of Aluminum. Neat huh?