Question

UMM.. I keep coming up with the wrong answer? How do I calculate this?
1. Calculate the number of Li atoms in 9.9 moles of Li.
Express your answer using two significant figures.
2. Calculate the number of CO2 molecules in 3.13×10−2 mole of CO2.

3.Calculate the moles of Cu in 2.4×1021 atoms of Cu.

4.Calculate the moles of C2H6 in 8.60×1023 molecules of C2H6.

Answer 1

There are 6.02E23 atoms in a mole of atoms or 6.02E23 molecules in a mole o molecules; actually, 6.02E23 anythings in a mole of anythings. Just as there are 12 in a DOZEN, 144 in a GROSS, 500 sheets in a REAM of paper,etc, there are 6.02E23 molecules in a mole. It's just another unit like dozen or gross. From your previous work it appears you like to work with unit factors so I'll stick with that.
a. 9.9 mol Li x (6.02E23 atoms/mol) = ? atoms Li.
b. 3.13E-2 mol CO2 x (6.02E23 molecules/mol) = ? molecules CO2
c. 2.4E21 atoms Cu x (1 mol Cu/6.02E23 atoms) = ? mols Cu
d. etc.

You will note that all of these are alike; they consist of taking the number you are given and multiplying by a factor. It's up to you to supply the factor. In this case it's (6.02E23 atoms/1 mol) or (6.02E23 molecules/1 mol) or (1 mol/6.02E23 whatevers)

Answer 2

I'm doing things on Mastering Chemistry dot com my homework.. I thought i was doing it right.. I was doing 9.9 * (6.02E23/1) It didn't like my significant figures.. It was wanting 6.0 * 10^24 as the answer, not 5.95 ugh... Thanks Dr. Bob.. Will probably be posting more for help! this stuff is hard, but you are a great help!!