Finding exact values for empirical formula calculations?

[ipodtouch]

I just wanted to confirm with the board if we should do all long division by hand and come up with EXACT values for empirical formula problems.


I was wondering because this seems to take time, but this is the way BR definitely is instructing us to.

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[chiddler]

example question, please. i don't remember doing long division.

 

[ipodtouch]

An unknown stable gas is composed of 13.10%H, 52.23%C, and the remainder O.
A 0.10 mole sample weights 4.61 grams. What is the molecular formula for the compound?

A.) C2H6O
B.) C3H8O
C.) C3H9O
D.) C4H8O


I thought about trying to solve this with number estimation, but couldn't come up with figure that would solve. So I looked at the solutions and they pretty much did straight up long division and rounded to the nearest 10th.

 

[SaintJude]

The Answer is A. For me, empirical formula question are quickest to do by looking at the answer choices--MCAT is multiple choice & this is a case where we can actually use that to our advantage!

If 0.1 mole is 4.6--then 1 mole is 46 grams.

A. ) 24 + 6 + 16 = 46 Bingo. Done. Took 40 seconds. Badaboom. badabing..ok, i'll stop.

 

[chiddler]

The Answer is A. For me, empirical formula question are quickest to do by looking at the answer choices--MCAT is multiple choice & this is a case where we can actually use that to our advantage!

If 0.1 mole is 4.6--then 1 mole is 46 grams.

A. ) 24 + 6 + 16 = 46 Bingo. Done. Took 40 seconds. Badaboom. badabing..ok, i'll stop.

yesh! alternatively

i noticed that it's 50% carbon. all the compounds with 3+ carbons have more than 50% carbon.

banana boom

 

[MedPR]

An unknown stable gas is composed of 13.10%H, 52.23%C, and the remainder O.
A 0.10 mole sample weights 4.61 grams. What is the molecular formula for the compound?

A.) C2H6O
B.) C3H8O
C.) C3H9O
D.) C4H8O


I thought about trying to solve this with number estimation, but couldn't come up with figure that would solve. So I looked at the solutions and they pretty much did straight up long division and rounded to the nearest 10th.

I'm sure they also gave you a quicker, more MCAT friendly, way to answer this question. I'm just about finished with content review and nowhere in TBR did I see them ever reccommending long division. As SaintJude said, if 0.1mol 4.61 grams, then 1mol is 46 grams. 2C + 6H + 1O = 24 + 6 + 16 = 46.

Say they hadn't given you the information that 0.1mol=4.61grams.

13% is H
52% is C
100-(13+52)=35% is O

C = 12
O = 16
H = 1

52/12 = a bit more than 4
13/1 = 13
35/16 = a bit more than 2

So your formula will have a bit more than 4 carbons, 13 hydrogens, and a bit more than 2 oxygens.

C4H13O2 = C2H6O

Again, no long division necessary.