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Topic: Need Help solving for X and Y in a molecule  (Read 4655 times)

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Offline Joules23

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Need Help solving for X and Y in a molecule
« on: October 24, 2006, 09:31:15 PM »


So far i have:
Cx(H2O)y + O2-> CO2 + H2O

im pretty sure this is wrong.. but:
.9999/(12.01x + 18.02y) + (1.12/32) = (1.5427/44.01) + (.5789/18.02)

i tried completeing that but it comes out to zero
« Last Edit: October 24, 2006, 10:53:53 PM by Joules23 »

Offline AWK

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Re: Need Help solving for X and Y in a molecule
« Reply #1 on: October 25, 2006, 03:00:19 AM »
Cx(H2O)y + (x+y/2)O2-> xCO2 + yH2O
Calculate masses of C in CO2 and  H in H2O. The rest of mass of your sample is an oxygen.
Change masses to moles and derive an empirical formula. Finally find how many emirical formulas consist in a molecular mass.
AWK

Offline Joules23

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Re: Need Help solving for X and Y in a molecule
« Reply #2 on: October 25, 2006, 09:02:22 AM »
ok im kinda lost, let me break down your explanation:

Step one:
Calculate masses of C in CO2 and  H in H2O. The rest of mass of your sample is an oxygen
mass of C = .420987
mass of H = .06476
mass of O = 1.6358

Step two:
Change masses to moles
.03505 mol C
.06476 mol H
.03408 mol O

Step three:
derive an empirical formula.
Heres where im not sure.. divide each mol, by whatever my smallest mol is? like this:
.03505/.03408 = 1
.06476/.03408 = 1.9 = 2
.03408/.03408 = 1

Step four:
find how many emirical formulas consist in a molecular mass.
Now here i have no idea how to do this

Offline AWK

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Re: Need Help solving for X and Y in a molecule
« Reply #3 on: October 25, 2006, 09:59:52 AM »
Mass of oxygen = 1.000 - 0.4210 - 0.0648 = 0.5142 not 1.6358
that gives 0.0321 moles of oxygen
molar ratio C : H2O = 0.03507 : 0.03216
From this numbers you can see that compound should contain slightly more carbon atoms than water molecules
Tentatively , assuming 1:1 stoichiometry you need 11.4 times formula of CH2O. Try to fit 12:11 12:10 and so on and calculate MW for each trial. You will find the best fit  C:H20 = 12:11
And in this case the empirical formula is equal to the molecular formula.
AWK

Offline Joules23

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Re: Need Help solving for X and Y in a molecule
« Reply #4 on: October 25, 2006, 10:35:56 AM »
so the unit formula is:
C_12(H_2O)_11
?
its funny how my teacher never even talked about this and now expects us to know how to do this type of problem.. this is supposed to be an intro to chem class... now i see why he has a rating of a 1 (the worst possible)

Offline enahs

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Re: Need Help solving for X and Y in a molecule
« Reply #5 on: October 25, 2006, 04:03:09 PM »
Quote
its funny how my teacher never even talked about this and now expects us to know how to do this type of problem.. this is supposed to be an intro to chem class...

Really, it is much simpler then all that, just using your basic knowledge.
The only place for the C to come from in the CO2 is from the carbohydrate, and the only place for the hydrogens in the water to come from is the carbohydrates.

So just calculate the moles of the product, 0.0350 CO2 (and therefore C in carbohydrate) and 0.0321 H2O (and therefore H2O in the carbohydrate, not because both are H2O but because both have hydrogen and the same stoichimoetry).

Then you have the mole ratio just like many other problems you have probably done:

0.0350 Mol C        =    1.09 Mol C      
0.0321 Mol H2                1 Mol H2

And then to get 1.09 to a even number you multiply times 11 to get 12, and so then you have to multiply 1 mole of H2 (and therefore the H2O in the carbohydrate) by 11, to get the C12(H2O)11.

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