It's a simplistic question (review) which I simply forgot.

1) Determine the molecular formula of a compound containing 80% carbon and 20% hydrogen. Molar Mass is 30.0g/mol

2) Determine the percentage composition of sodium sulfate (Na2SO4)

3) A 1.22gram sample of liquid is vaporized at 95 degrees celcius and 102 kPa. The sample occupies 218mL (0.218L). If the percentage composition of this compound is 71.4% Carbon, 9.59% Hydrogen, and 19.0% Oxygen, what is its molecular formula.


If you can tell me how you do it, or how to do it, that would be highly apprecaited.

1. Take percent to be a whole #: 80%=80
Divide this number by weight of one Carbon (12.00)
80/12=6.67
Do same for Hydrogen
20/1.0079=19.84
Divide each number through by the smallest
6.67/6.67=1
19.84/6.67=2.97 (round to 3)
Molecular formula= CH3, which happens to be methyl.

2. Find weight of each of these.
Na=22.99
S=32.066
0=16.00
It's Na2SO4, so multiply the Na by 2, and the O by 4.
Now,
Na=45.98
S=32.066
O=64.00
Add them up: 142.046
Now divide each number by the total
Na: 45.98/124.046= 32.37% <---- this is the percent composition of Na.
S: 32.066/142.046=22.57% " "
O: 64.00/142.046= 45.06%

I'll let you do the last one, hope you get the picture. Keep in mind for #3 you're using a 1.22 gram sample, so adjust your calculations accordingly. Don't let that 95 Celcius and 102 kPa or sample occupancy through you off- you don't need that.
(I got C4H8O for an answer, but that's without changing the calculations to fit the sample size- normally you assume sample size of 100 grams, but in this case they tell you, so use 1.22 grams instead.)

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