How many molecules are there in 85 g of  carbon tetrachloride?

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ncchemist | eNotes Employee

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You are only allowed to ask one question at a time so I edited down accordingly.  You can use the same reasoning here for the other question as well.

We are asked to find the number of molecules there are in 85 grams of carbon tetrachloride (CCl4).  To do this we need to know two things.  First, how many moles of CCl4 there are in 85 grams. And second, how many molecules there are in each mole.

The number of moles can be easily calculated by figuring out the molecular weight of CCl4 and then dividing the two numbers.  The molecular weight of CCl4 can be found by adding the atomic weights of the different elements present as found on the periodic table.  One molecule of CCl4 has 1 carbon atom and 4 chlorine atoms.  The atomic weight of carbon is 12.01 and the atomic weight of chlorine is 35.45.  So the molecular weight of CCl4 is 12.01 + 4*35.45 = 153.81 g/mol.

The number of molecules in a mole of any given substance is always the same: 6.022 x 10^23.  This is a defined number that is called Avogadro's number.  So multiply the number of moles by Avogadro's number to get the number of molecules.

We can now put this all together as shown below.  Notice that the units cancel to give the desired quantity (note that the word molecules is abbreviated as "molc":

`85 g * (mol)/(153.81 g) * (6.022x10^23 molc)/(mol) = 3.328 x 10^23 molc`

So the answer is 3.328 x 10^23 molecules of CCl4.