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### Topic: Question about percentage by mass  (Read 3810 times)

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#### antoinetta

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##### Question about percentage by mass
« on: September 24, 2006, 10:14:03 PM »
Hi, I'm just having trouble setting up this question:  a mixture consisting of C4H6O6 with C6H8O7 was analysed to determine its elemental composition.  Combustion of a 299.0mg sample produced 384.4 mg of CO2 and 110.2 mg of H20.  What is the percentage by mass of hydrogen?  What is the percentage by mass of the C4H6O6?
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#### Bakegaku

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##### Re: Question about percentage by mass
« Reply #1 on: September 24, 2006, 10:45:41 PM »
first I'd suggest finding the percent mass in the original sample of hydrogen by finding the mass of the hydrogen in the resultant water, then dividing that number by 299.0.  Once you have that number, you'd find the percent of hydrogen in each of the compounds and write equations that would have it average out to the hydrogen in the whole sample.

That's a fairly vague explanation, but hopefully it will lead you in the right direction.  Naturally, if you have a question, ask.
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#### antoinetta

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##### Re: Question about percentage by mass
« Reply #2 on: September 25, 2006, 12:51:41 AM »
So following your suggestions:  I have found that the % Carbon is 35.09%, Hydrogen is 4.124%, and Oxygen is 60.79%.  So, how do I find out the percentage of C4H6O6?  I know that I have set up two equations and two variables (ie x and y).  X and Y have a total mass of 0.299 g ... so now what?
The oldest, shortest words - "yes" and "no" are those which require the most thoughts.  - Pythagoras

#### AWK

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##### Re: Question about percentage by mass
« Reply #3 on: September 25, 2006, 04:09:32 AM »
Set:
A=moles of C4H6O6
B=moles of C6H8O7
Calculate molecular masses for these compounds, and moreover, for CO2 and H2O.
Set system of equation:
MM_CO2(4A+6B)=0.3844 (1 mole od A forms 4 mole of CO2 and 1 mole of B forms 6 moles of CO2)
MM_H2O(6A/2+8B/2)=0.1102 (denominator 2 is because 2 H atoms forms 1 molecule of water)
Miligrams were changed to grams because we use MMs in grams.

Then mass of sample is equal to A x MM_of_C4H6O6 + B x MM_of_C6H8O7 and mass percentage of the sample is easy to calculate.
Mass percentage_of_H = moles_of H2O x (atomic_mass_of H x 2) / mass_of sample
AWK