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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: kimi85 on August 03, 2007, 05:47:19 AM

Title: gravimetric analyses
Post by: kimi85 on August 03, 2007, 05:47:19 AM

1)A 0.8000-g sample of iron ore containing 21.00% Fe is analyzed gravimetrically.  If the final precipitate (assumed to be Fe²O₃) contains 20.0 mg of Al2O3 by mistake, what percentage of Fe does the analyst find?



2) A 0.5000-g sample that contains KCl and KI gives a precipitate of AgCl and AgI that weighs 0.2818 g.  A 0.8000-g sample is titrated with 0.1053 M AgNO₃, requiring 26.74 ml.  Calculate the percentages of KCl and KI in the sample.

Thanks!

Title: Re: gravimetric analyses
Post by: AWK on August 03, 2007, 06:37:23 AM

Show your attempts

Title: Re: gravimetric analyses
Post by: kimi85 on August 03, 2007, 09:25:58 AM

For number 1, what I did is I first converted the 21% Fe to grams then to grams of Fe2O3..What I got is 0.240 grams..then I converted  0.020 g Al2O3 to grams of Fe2O3..Then I subtracted it from the 0.240 grams..then I calculated the percent Fe in the sample..

My answer is 18.24%.. Is that correct?

Title: Re: gravimetric analyses
Post by: kimi85 on August 03, 2007, 09:33:51 AM

For # 2,

x KCl  (mwAgCl/mwKCl) + 0.5 - x (mw AgI/mwKI)  = 0.2818 g

Then solve for x then calculate the percent..

But I don't know if the titration part is needed for the calculation..Is it just a confirmation part?

Title: Re: gravimetric analyses
Post by: AWK on August 06, 2007, 04:16:32 AM

Quote from: liemei on August 03, 2007, 09:33:51 AM

For # 2,

x KCl  (mwAgCl/mwKCl) + 0.5 - x (mw AgI/mwKI)  = 0.2818 g

Then solve for x then calculate the percent..

But I don't know if the titration part is needed for the calculation..Is it just a confirmation part?

If your sample is pure, then mass of AgCl + mass of AgI should be greater then mass of KCl+KI. Then you have an excessing variable in your first equation since mass of KCl+KI is in fact unknown, but much less then 0.5 g.
Titration gives you number of moles KCl+KI.

Title: Re: gravimetric analyses
Post by: kimi85 on August 06, 2007, 07:19:11 AM

Thank you..So i should find the difference?

Title: Re: gravimetric analyses
Post by: AWK on August 06, 2007, 08:39:39 AM

No, you should set a system of two equations with 2 variables based on moles of KCl (n1) and KI (n2).

n1+n2=0.1053x0.02674
then rescale it to 0.5 g
n1+n2=0.1053x0.02674x0.625 (volume in liters, 0.625=0.5/0.8)
Now you can compare this result to precipitation reaction
n1xMM(AgCl)+ n2xMM(AgI)=0.2818
and after finding n1 and n2 you can calculate coresponding masses followed by percentages.

Of course, there are other possibilities of choosing variables (masses or even percentages of KCl and KI). Try to set corresponding equations.

Title: Re: gravimetric analyses
Post by: kimi85 on August 06, 2007, 09:54:31 AM

thank you very much!