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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Slumdawg on February 28, 2010, 09:54:49 am

Title: Gravimetric Questions HELP
Post by: Slumdawg on February 28, 2010, 09:54:49 am: This may be a really easy question for most but I can't seem to get it right. Thanks to anyone who answers this, greatly appreciated. Could you please show workings out and the formulas used for each step i.e. n = m/Mr

Q1 A sample of limestone is analysed for its calcium carbonate content as follows. A 25.00g sample is crushed and heated to a high temperature. It is then cooled, weighed and reheated until a constant mass is obtained. The mass remaining at the end of this process is 11.64g.
Use the results obtained above to calculate the mass of calcium carbonate in the limestone, and hence its percentage purity.
Answer in the back of the book: 95%

Q2 Phosphorus is present in many fertilisers. It can be determined gravimetrically by carrying out a number of steps, eventually leading to its precipitation as insoluble Mg2P2O7. In one such analysis, a 14.298g sample of fertiliser yielded 4.107g of Mg2P2O7 precipitate.
a). Calculate the mass of phosphorus in the precipitate
Answer in the back of the book: 0.8967g
b). Hence calculate the percentage of phosphorus in the fertiliser.
Answer in the back of the book: 6.271%
Title: Re: Gravimetric Questions HELP
Post by: Edmund on February 28, 2010, 10:28:26 am: Sorry if these doesn't help, this is how I would have done it but I can't see where I'm going wrong...

Q1

$CaCO_3 (s) \rightarrow CaO (s) + CO_2 (g)$

When limestone is heated, carbon dioxide is given off and calcium oxide remains. CaO is a solid at room temperature (quicklime).

$n(CaO)=\frac{11.64g}{56.1g/mol}=0.2075mol$

$n(CaCO_3)=n(CaO)=0.2075mol$

$\therefore m(CaCO_3)=n(CaCO_3)*M(CaCO_3)=0.2075*100.1=20.77g$

$\frac{20.77}{25}*100=83$

Q2

$n(Mg_2P_2O_7)=\frac{4.107g}{222.6g/mol}=0.01845mol$

$n(P)=2*n(Mg_2P_2O_7)=2*0.01845mol=0.0369mol$

$m(P)=0.0369*31=1.1439g$
Title: Re: Gravimetric Questions HELP
Post by: Slumdawg on February 28, 2010, 07:04:01 pm: Thanks for trying mate. If anyone else gets it I will be forever in their debt haha
Title: Re: Gravimetric Questions HELP
Post by: superflya on February 28, 2010, 07:11:33 pm: im getting the same answers as edmund :S
Title: Re: Gravimetric Questions HELP
Post by: moneran on February 28, 2010, 10:26:15 pm: Yea i got the same answer even tryied to assume that the original amount of CaCO₃ had an amount of water in it although this would make it a lower percentage ....... so yea 95% got me bust
For the secound question i got the percentage of P in the Mg₂P₂O₇ * by mass of precipitate ie 4.107g which gave 1.1428 grams which is amount of P in the 4.107 gram substance.
Title: Re: Gravimetric Questions HELP
Post by: Mao on March 01, 2010, 11:29:23 pm: Quote from: shilayli06 on February 28, 2010, 07:04:01 pm
Thanks for trying mate. If anyone else gets it I will be forever in their debt haha

Haha, forever in debt over a couple Chemistry questions?

I can confirm with the general consensus that the suggested solutions provided by the book are wrong.
Title: Re: Gravimetric Questions HELP
Post by: Slumdawg on March 02, 2010, 09:28:33 pm: Well at least it puts my mind at ease. Feel like I'm the only one who doesn't get the right answer all the time.
I'll check with my teacher tomorrow, he might even figure it out. Although he scares me too much so I may not go to him haha.

And to Mao. Looks like I'm not going to be in anybody's debt forever because nobody could answer every question haha.

Thanks to everyone for what they've contributed.