Chemistry

Ok, I'm trying my best here, I'm learning, and I'm thinking I'm on a roll, until I can't get the right answer again.....

First.. the butane question

Calculate the mass of Butane needed to produce 82.7g Carbon dioxide.

2C4H10 +13 O2--->10H2O +8CO2

Ratio is 2 C4H10 to 8 CO2
82.7 g/molar mass of CO2 44 = 1.88g/mol
but it's asking about butane
1.88 ( 2/8) = .47 and then .47 *58.12 = 27.3 I figured it out.. got the right answer .. But now on the next question it's asking for how many grams.. I'm confusing myself??

4Na+O2-->2Na2O

How many grams of Na2O are produced when 19.2 g. of Na reacts?
I started by getting moles..
19.2/22.9=.84 mols Na
.84(4/2)=1.68 then I did 1.68 *61.8 (molar mass of Na2O) and that was not the right answer... UGH!!! the right answer is 25.9 g somehow, and I can't figure out how.?

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  1. You stuck in numbers with the Na2O question without showing the units and you reversed the factor. The units are not there and that's why you can't find the error.
    First I would not have rounded the 0.84 but kept 0.838. Why throw a perfectly good digit away. You have three digits in 19.2 so you should keep at least 3 in the answer. In fact, I usually keep an extra digit (as a guard digit) so I just leave that value in my calculator as 0.8384 and multiply that by the conversion factor.
    0.8384 mols Na x (2 mols Na2O/4 molls Na) and that x (62 g Na2O/mol Na2O = about 26 grams. THEN you count the digits and round to the right number of significant figures.

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