Question #b171f
1 Answer
O.K., you can do this. It just takes thinking about it in a little different way than you are, perhaps, used to:
First, you need to know how much copper(II) sulfate pentahydrate you have got to start with.
You know that the total mass is 14.98 grams, and that of that 14.98 grams, 10.00 grams is the mass of the crucible. So.....
Right! the mass of the copper compound is 14.98 - 10.00 = 4.98 grams.
So, having that number in hand, let's figure out how many moles of copper (II) sulfate pentahydrate we have. We need to know moles because each element has a different mass, but elements react or form compounds by reacting in set ratios of atoms (or moles). So let's figure out how many moles of the copper (II) sulfate pentahydrate we have:
Copper has an atomic mass of 63.546. Sulfur is 32.066. Oxygen is 15.9994, and hydrogen is 1.0079. So copper (II) sulfate pentahydrate has a molar mass of:
(63.546 x 1) + (32.066 x 1) + (15.9994 x 4) + (1.0079 x 10) + (15.9994 x 5) = 249.6856 g/mol.
We have 4.98 grams of the compound, so
O.K., we've gotten the starting part done! Now let's see how much water we have lost.
Wait! Is it really that easy? We lost (14.98 - 13.54) grams of water? Yup! It really is that easy! We lost 1.44 grams of water.
Ah, but again, we need to convert the mass of water lost to the number of moles of water lost:
So for 0.019945 moles of copper (II) sulfate pentahydrate we lost 0.0799298 moles of water or
To summarize what we did: We started by figuring out how much copper (II) sulfate pentahydrate we had, converted that to moles, then figured out how much water we had lost, converted that to moles, and finally found the ratio of moles of water lost to moles of copper (II) sulfate pentahydrate.
Got it? O.K., now go find a similar problem and do it by yourself!
Here is a video discussing how to find the empirical formula of copper sulfate.
