Hello, Brooke!
Any time that you see the words "grams" and "moles" in a question, you need to remember that you will need molar mass. Molar mass is the amount of grams of 1 mole of a substance.
To find molar mass, you just need to find the masses from the periodic table! So before beginning the calculations, it would be a good idea to find the molar mass using the periodic table.
CF2Cl2
C = 12.01
F = 18.99 *2 = 37.98 (I multiply F by 2 because there are TWO fluorine atoms in the compound)
Cl = 35.45 *2 = 70.9 (I multiply Cl by 2 because there are TWO chlorine atoms in the compound)
--------------------------------
12.01 + 37.98 + 70.9 = 120.89 g/mol **The unit for molar mass is always grams per mole**
So now that we have the molar mass, now what? We always start with what is given, which is in the question.
In the question, we are given 1.44 kg. But our molar mass is in grams! This tells me that we will have to convert kg to g.
1.44 kg | 1000 g
-------------------------- (the short, vertical bars are multiplication and the long horizontal bar is division)
| 1 kg I put kg at the bottom to cancel the kg and put grams on the top. There are
1000 g in a kg, so I put 1000 by the grams.
**NOTE: At this point, some people may choose to go ahead and multiply by 1.44
by 1000. I do not do that; I prefer to calculate everything all at once. I will
finish setting up the problem and then do the calculation**
Now that we have canceled kilograms and are left with grams, we're still not done! The question wants moles. To find moles, I will have to use the molar mass at this point.
**HINT: The mass that you calculate will ALWAYS go with "grams" and never with moles! It always grams per 1 mole**
1.44 kg | 1000 g | 1 mol
--------------------------------------- (I put grams at the bottom to cancel the grams on top)
| 1 kg |120.89 g (Remember, the molar mass we found always goes with grams)
Now, I can multiply all of the top numbers together and then divide by the denominators
(1.44 *1000)/ 120.89 = 11.91165 mol
Now, time to apply significant figures. The given only had 3 significant figures, so the final answer should be 11.9 mol
11.9 mol
