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Author Topic: need help with chem AS  (Read 2315 times)

Offline ksitna

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need help with chem AS
« on: January 12, 2010, 04:04:38 pm »
17.1 g of aluminium sulfate, Al2(SO4)3, was dissolved in water.
Calculate the number of sulfate ions, SO4
2–, present in the solution formed.
[Assume the molar mass of Al2(SO4)3 is 342 g mol–1 and the Avogadro Constant is
6 × 1023 mol–1.]

how do u do this
can u help me pleassee
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01chris

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Re: need help with chem AS
« Reply #1 on: January 12, 2010, 06:13:50 pm »
The formula of the compound Al2(SO4)3 tells you that you have 3 SO4 ions for every molecule of the compound.

You need to work out how many moles of the compound you have: n = m/M  -or as I like to remember it- no. moles = grams/RAM.

17.1g/342g mol-1 = 0.05mol.

Avagadro's number says that a mole of a substance = 6x1023 particles. You have 0.05 mol therefore 0.05 x 6x1023 particles (which equals 3x1022 ).

Going back to what we said at the start: for every Al2(SO4)3 molecule you have 3 SO4 ions. You have 3x1022 Al2(SO4)3 therefore you have 3 x 3x1022 SO4 ions (= 9x1022 ).

Does that help?
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Offline Dento

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Re: need help with chem AS
« Reply #2 on: January 12, 2010, 06:16:59 pm »
hey ksitna,
here's how to deal with these questions,

first, find the numer of moles of the sulphate ions which is >>>> 17.1/342 = 0.05 moles

then multply this mole number with the avogadro's constant and 3.    ( 3 is the number of sulphates given in the question >> Al2(SO4)"3" )

so, it goes this way,



moles of sulphate = 0.05 moles
number of sulphate ions present = 0.05 x (6x10 raised 23) x 3
= 9x10 raised 22
Answer is "D"

this is the final answer, hope i helped u, if there is anything else, feel free to ask
good luck this thursday !!
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Offline ksitna

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Re: need help with chem AS
« Reply #3 on: January 12, 2010, 06:34:56 pm »
THANK UUU!!!!!!!
I OWE U GUYS ONE
THANKS A LOOTTTT ;D
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Offline ksitna

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Re: need help with chem AS
« Reply #4 on: January 12, 2010, 06:37:02 pm »
sooo sorry to be a bother
AGAIN
but um are the oxidation stuff and flame tests required for this module?
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01chris

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Re: need help with chem AS
« Reply #5 on: January 12, 2010, 06:49:04 pm »
No worries! Glad I could help.

I'm afraid I can't answer your other question - I'm doing a different board (OCR). Sorry
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Offline ksitna

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Re: need help with chem AS
« Reply #6 on: January 12, 2010, 06:51:13 pm »
Quote from: 01chris on January 12, 2010, 06:49:04 pm
No worries! Glad I could help.

I'm afraid I can't answer your other question - I'm doing a different board (OCR). Sorry


ohh!!
no prob
thanks anywayy :D
really appreciated it
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Offline mountaindewaddict

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Re: need help with chem AS
« Reply #7 on: January 12, 2010, 06:54:51 pm »
No flame tests or oxidation in unit 1.
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Offline sweetie

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Re: need help with chem AS
« Reply #8 on: February 21, 2010, 07:38:57 pm »
Hey guyz,
 doubt in organic chemistry P1

Q1) which compound is unaffected by hot alkaline potassium manganate (VII) and gives hydrogen when treated with sodium?
a.(CH3)2CHCOCH3
b.(CH3)3COH
c.CH3CH2CH(OH)CH3
d.CH3CO2CH(CH3)2

I thought da answer was C but it is wrong  :'( dunno why!!!!!

I really appreciate any help from u guyz  :-*
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nid404

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Re: need help with chem AS
« Reply #9 on: February 21, 2010, 08:23:29 pm »
See b is a primary alcohol which can get oxidised, c is a secondary alcohol which can also get oxidised
so it leaves us with a and d
a is a ketone, which won't be affected by KMNO4...it won't get oxidised further
so it leaves us with d....
By the way what does the ms say?...cuz i keep forgetting reaction mechanisms...so i may have messed up
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Offline sweetie

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Re: need help with chem AS
« Reply #10 on: February 22, 2010, 05:06:26 pm »
ya i guess u messed up a bit , the ms says "B"
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nid404

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Re: need help with chem AS
« Reply #11 on: February 22, 2010, 05:44:43 pm »
yaa....stupid me....I jst realized...

sorry
B is actually 2 methyl, propan-2-ol
This won't be affected by KMNO4 because it a tertiary alcohol....
alcohols=water in organic chem....so it will give hydrogen with sodium too
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Offline sweetie

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Re: need help with chem AS
« Reply #12 on: February 22, 2010, 05:52:12 pm »
why is it not "c"
It is also an alcohol and no double bond, so no reaction with KMnO4?
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nid404

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Re: need help with chem AS
« Reply #13 on: February 22, 2010, 05:55:21 pm »
Quote from: notty_me on February 22, 2010, 05:52:12 pm
why is it not "c"
It is also an alcohol and no double bond, so no reaction with KMnO4?


C is a secondary alcohol...it will form a ketone when oxidised

unfortunately i'd mistaken B for a primary alcohol...not noticing it was (CH3)3
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Offline sweetie

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Re: need help with chem AS
« Reply #14 on: February 22, 2010, 06:07:08 pm »
got it  ;D

thanx  :-*
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