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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jen501 on January 18, 2005, 04:29:35 PM

Title: elemental composition of a binary mixture
Post by: jen501 on January 18, 2005, 04:29:35 PM

A white powder, consisting of a simple mixture of tartaric acid (C4H6O6) and citric acid (C6H8O7) was analysed to determine the elemental composition. Combustion of a 607.8-mg sample produced 800.6 mg of CO2 and 225.4 mg of H2O.
Use atomic masses: C 12.011; H 1.00794; O 15.9994.



Calculate the % carbon, by mass, in the sample.


i tried calculating the mass of carbon and dividing it by the sample but i do not get the answer...can anyone give any help?

Title: Re:elemental composition of a binary mixture
Post by: Mr Amino on January 18, 2005, 11:10:38 PM

I get 218.5 mg of C in the sample. Is that what you got?

Title: Re:elemental composition of a binary mixture
Post by: AWK on January 19, 2005, 01:12:50 AM

You obtained mass of carbon in sample of  607.8 mg correctly. Now calculate carbon percentage in the sample.

Title: Re:elemental composition of a binary mixture
Post by: sundrops on January 21, 2005, 02:25:46 AM

i have a very similar problem - how did u do your calculations mr amino?

Title: Re:elemental composition of a binary mixture
Post by: Donaldson Tan on January 21, 2005, 10:30:34 PM

all carbon in carbon dioxide formed during the complete combustion comes from the sample. therefore no. of moles of carbon in CO2 i= no. of moles of carbon in the sample.

all hydrogen in water formed from the complete combistions comes from the sample as well. therefore no. of moles of hydrogen in sample = no. of moles of hydrogen in the water.

since the number of moles of carbon and hydrogen is known in the sample, deducting their corresponding mass from the sample mass gives you the mass of oxygen in the sample, and thus dividing this mass by the oxygen's molar mass, you will find the number of moles of oxygen in the sample.

Title: Re:elemental composition of a binary mixture
Post by: sundrops on January 22, 2005, 05:52:00 AM

AWESOME!! I found the % carbon, % hydrogen and % oxygen in the sample!  ;D

now all I need is to calculate the % citric acid (C6H8O7), by mass in the sample

how would I go about doing that?  

Title: Re:elemental composition of a binary mixture
Post by: sundrops on January 22, 2005, 06:00:05 AM

my question is essentialls identical to jen501's except that mine uses different values. How would I try to determine the percent of citric acid within the sample?

Title: Re:elemental composition of a binary mixture
Post by: AWK on January 22, 2005, 08:14:37 AM

Calculate mass percentage of elements for citric and tartaric acids and solve system of equations
p1 + p2 = 100 where p1 and p2 are percentages of both acids (unknowns) ; 1and 2 denote these acids, respectively.
cp1 x pC1 + cp2 x pC2 =100 x pC_total (pC means percentage of carbon)

Title: Re:elemental composition of a binary mixture
Post by: sundrops on January 22, 2005, 09:36:57 AM

I'm not really sure what you mean is it like:

x(% mass carbon in tartaric) + (1-x)(% mass carbon in citric acid)

where x = is the fraction of tartaric acid.

but then how do I know what athat fraction is?

Title: Re:elemental composition of a binary mixture
Post by: sundrops on January 22, 2005, 11:01:15 PM

got it figured out - thanks all for the *delete me*  :D

Title: Re:elemental composition of a binary mixture
Post by: Mitch on January 22, 2005, 11:59:11 PM

Scooby Snacks are good rewards. ;)

Title: Re:elemental composition of a binary mixture
Post by: imclair on May 10, 2008, 01:58:44 PM

Quote from: AWK on January 22, 2005, 08:14:37 AM

Calculate mass percentage of elements for citric and tartaric acids and solve system of equations
p1 + p2 = 100 where p1 and p2 are percentages of both acids (unknowns) ; 1and 2 denote these acids, respectively.
cp1 x pC1 + cp2 x pC2 =100 x pC_total (pC means percentage of carbon)

what's cp1 ?

Title: Re: elemental composition of a binary mixture
Post by: Borek on May 10, 2008, 02:05:52 PM

Looks like AWK meant p1 when he wrote cp1.

Scooby snack for searching forums and finding an old thread that fits your problem :)