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Author Topic: CHEM AND BIO HELP AND TIPS HERE  (Read 350797 times)

Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2325 on: May 31, 2010, 11:25:31 am »
Quote from: aangel42 on May 31, 2010, 07:03:17 am
also, guys, don't know why my computer's decided to be all retarded today, but i'm not able to download the "questions paper 1 from 1994-2004" on this link: http://freeexampapers.com/Dndex.php?d=SUdDU0UvQmlvbG9neS9DSUUvMTk5MyAtIDIwMDM=
or the answers to it (first file)
it's for biology, and my computer keeps saying that it's a reported malicious site or something.
please help?????!!!!!

OK, see if you can download them by clicking on these links.  If not, let me know.

Questions Paper 1 June 1994 -- 2004.pdf:
http://www.freeexampapers.us/IGCSE/Biology/CIE/1993%20-%202003/Questions%20Paper%201%20June%201994%20--%202004.pdf

Answers June 1993 -- 2003 P1, 3, & 6.pdf:
http://www.freeexampapers.us/IGCSE/Biology/CIE/1993%20-%202003/Answers%20June%201993%20--%202003%20P1,%203,%20&%206.pdf
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"In helping others, we shall help ourselves, for whatever good we give out completes the circle and comes back to us."

Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2326 on: May 31, 2010, 11:38:26 am »
Quote from: CatAly$t on May 31, 2010, 07:49:11 am
and please if somebody can do this... ???

OK, hopefully this clears your doubt.

This time, you'll need to also apply this formula:
                                                                      
Concentration (mol/dm3) = Moles / Volume (dm3)

So:

i) Number of moles of NaOH used = 2.24*0.025 = 0.056 mols

ii) Maximum number of moles of Na2SO4.10H2O that could be formed = From the equation given, you can see 2 moles of NaOH gives 1 mole of Na2SO4.10H2O.  So it is simply: 0.056/2 = 0.028 mols

iii) Mass of one mole of Na2SO4.10H2O = 322g

iv) Maximum yield of sodium sulphate-10-water = 0.028*322 = 9.02g

v) Percentage yield = (3.86/9.02)*100 = 42.8%

If you don't understand any of this, I'll be happy to explain ;)
« Last Edit: May 31, 2010, 11:43:12 am by Ivo »
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Always willing to help!  8)
"In helping others, we shall help ourselves, for whatever good we give out completes the circle and comes back to us."

Offline CatAly$t

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2327 on: May 31, 2010, 01:40:18 pm »
Quote from: Ivo on May 31, 2010, 11:38:26 am
OK, hopefully this clears your doubt.

This time, you'll need to also apply this formula:
                                                                      
Concentration (mol/dm3) = Moles / Volume (dm3)

So:

i) Number of moles of NaOH used = 2.24*0.025 = 0.056 mols

ii) Maximum number of moles of Na2SO4.10H2O that could be formed = From the equation given, you can see 2 moles of NaOH gives 1 mole of Na2SO4.10H2O.  So it is simply: 0.056/2 = 0.028 mols

iii) Mass of one mole of Na2SO4.10H2O = 322g

iv) Maximum yield of sodium sulphate-10-water = 0.028*322 = 9.02g

v) Percentage yield = (3.86/9.02)*100 = 42.8%

If you don't understand any of this, I'll be happy to explain ;)


double thanks man  ;D...

+rep twice on u.... ;)
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Offline Vin

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2328 on: May 31, 2010, 08:39:59 pm »
Okay guys you will kill me for this but i don't seem to get it .. I don't know why! :-\
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Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2329 on: May 31, 2010, 09:34:13 pm »
Quote from: ~VIN1094~ on May 31, 2010, 08:39:59 pm
Okay guys you will kill me for this but i don't seem to get it .. I don't know why! :-\

It's hard to give a concrete explanation, but we know it's between pH and temperature.  I would go for pH as I've seen in many textbooks that the effect of pH on the activity is more of a natural parabola than for temperature.  I suppose, when increasing temperature above the optimum temperature, the activity suddenly decreases more than if pH increased, I guess.

Sorry for giving you a slightly blurred explanation, but that's the best I can do, hope that helps a little :).  Consider the effect of temperature and pH below, and make your judgement ;)  This is a very good graph as the effects of the two variables are given side by side.
« Last Edit: May 31, 2010, 09:36:14 pm by Ivo »
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Offline Vin

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2330 on: May 31, 2010, 09:40:21 pm »
Ok thanks ..  I get it ;) :D
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Offline jellybeans

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2331 on: June 01, 2010, 04:35:47 am »
Hiiii, um there doesnt seem to be any answers to Q5 onwards in the MS for M/J 02.

http://freeexampapers.com/FreeExamPapers.com_.php?__lo=SUdDU0UvQ2hlbWlzdHJ5L0NJRS8yMDAyIEp1bi8wNjIwX3MwMl9tc18zLnBkZg==

i've tried xtreme papers as well..
:-\ help?

thaaanks
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Urich Von Conon

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2332 on: June 01, 2010, 05:06:46 am »
Look at the examiner's report ;)
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Offline jellybeans

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2333 on: June 01, 2010, 05:13:59 am »
Quote from: Curio on June 01, 2010, 05:06:46 am
Look at the examiner's report ;)

THANK YOU! ;D
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Offline CatAly$t

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2334 on: June 01, 2010, 08:37:04 am »
can somebody answer these questions and please tell me y?? :-\

thanks
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Offline mdwael

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2335 on: June 01, 2010, 09:55:16 am »
anybody knows some good website/sources to learn about moles in chemistry? or just chemistry in basic? I really messed up in physics I dont want the same to happen in chemistry.. :'(. Help greatly appreciated
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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2336 on: June 01, 2010, 09:57:17 am »
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Offline jellybeans

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2337 on: June 01, 2010, 10:24:11 am »
I've got a huuuuuuge doubt.
If Mole = Mass / RFM , and here, they're asking for the number of moles of sodium atoms reacted.
So to find the mole of 2Na .. isn't it 11.5/46 = 0.25 Mole?  :-\
And if it's only 11.5/23 , which equals to 0.5, isn't that only ONE atom of Na but we need two here so dont we multiply the answer by 2? D:
Help please..  ???
Thaaanks!
« Last Edit: June 01, 2010, 10:26:14 am by jellybeans »
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Offline MasterMath

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2338 on: June 01, 2010, 10:30:15 am »
Quote from: jellybeans on June 01, 2010, 10:24:11 am
I've got a huuuuuuge doubt.
If Mole = Mass / RFM , and here, they're asking for the number of moles of sodium atoms reacted.
So to find the mole of 2Na .. isn't it 11.5/46 = 0.25 Mole?  :-\
And if it's only 11.5/23 , which equals to 0.5, isn't that only ONE atom of Na but we need two here so dont we multiply the answer by 2? D:
Help please..  ???
Thaaanks!
But Moles of Na exist as mono atomic .. so each mole of Na has one atom of Na not 2.. so if you say you have 1 mole of Na , its the same as 1Mole of atoms of Na..Why did you divide by 46?
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Offline MasterMath

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2339 on: June 01, 2010, 10:32:59 am »
Just follow the equation:
2Na            +              S -------->     Na2S
46                              32                       
11.5                            10(but excess so we wont use this)
We know that 11.5g of Na reacted .. so just use the simple formula .. mass/RFM .. 11.5/23 = 0.5moles
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