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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: shinjo on April 23, 2011, 06:05:58 AM

Title: gravimetric methods of analysis
Post by: shinjo on April 23, 2011, 06:05:58 AM

What weight of pyrite ore (impure FeS2) must be taken for analysis so that the BaSO4 precipitate weight obtained will be equal to one-half that of the percent S in the sample?
 
that's all the given.. how then to solve this?

Title: Re: gravimetric methods of analysis
Post by: Borek on April 23, 2011, 06:57:57 AM

First, try to write the condition (mass of precipitate equal to half percent S in the sample) in the form of mathematical equation.

m_BaSO4 = ...

Title: Re: gravimetric methods of analysis
Post by: shinjo on April 23, 2011, 09:16:33 AM

thanks sir, then after? i really find it hard to start because there are no numbers involved in the problem..
if it's okay with you sir, would you please elaborate your explanation a little bit?

Title: Re: gravimetric methods of analysis
Post by: Borek on April 23, 2011, 04:51:31 PM

Don't sir me ;)

Solve using symbols first. I already showed you first symbol to use - mass of precipitate. Sooner or later you will need to
use molar masses of substances and stoichiometric coefficients of the reactions involved - these will be your numbers.

Title: Re: gravimetric methods of analysis
Post by: shinjo on April 24, 2011, 02:04:50 AM

Thanks Mr. Borek.. One last question.. after setting up the equation the next thing to do is to get the %S of the sample using percentage composition of compound.. i'm wondering if what compound of sample will i be use to get the %S. Is it the compound
FeSO4 ? or BaS2 ? or FeS2 ? or BaSO4 ?...

Title: Re: gravimetric methods of analysis
Post by: Borek on April 24, 2011, 04:18:50 AM

No idea what you are asking about. You weight BaSO₄, you calculate a percentage of FeS₂. Amount of BaSO₄ is in a stoichiometrical relation to FeS₂.

Title: Re: gravimetric methods of analysis
Post by: shinjo on April 24, 2011, 04:30:58 AM

i still don't get it..sorry   ???, you mean i have to get first %FeS2? i thought first thing to do is to get %S then from the equation
m_BaSo4=1/2(%S) i could now get the mass/weight of BaSO4... so, basically i really have to get first %FeS2 but how? :-\

Title: Re: gravimetric methods of analysis
Post by: Borek on April 24, 2011, 05:08:00 PM

Assume you know mass of the precipitate (m_BaSO4) and mass of the sample (m_sample). Can you write equation that will use these two quantities to give S percentage in the sample?