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Originally posted by Flying grenade:
NaBD4 or NaBH4 need to be in anhydrous conditions right???
https://www.dropbox.com/s/vjp7sikfsz4zytr/20160928_114100.jpg?dl=0
Yes, but ethanol solvent can. -
ok got it. then D2O(in the qn) considered heavy water right???
D2O like H2O then NaBD4 or NaBH4 will undergo hydrolysis anot?
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photo of qn , 2004 p3 qn 3either bii)
https://www.dropbox.com/s/nzhs6tqnlys1r3s/20160928_124011.jpg?dl=0
for the 2011 buffer qn can find all the pH .
for this qn , is it possible to find pH of end point?? i dk the number of moles, as they didnt give initial vol of ethanoic acid, and hence i cannot find the no. of moles of ch3co2- and hence the conc of ch3co2- and hence cant find pH and end pt is it??
https://www.dropbox.com/s/ca7s5ylc59mgnwh/20160928_124018.jpg?dl=0
eh sorry. qn have give 20cm3. eyes didnt see properly just now
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Originally posted by Flying grenade:
ok got it. then D2O(in the qn) considered heavy water right???
D2O like H2O then NaBD4 or NaBH4 will undergo hydrolysis anot?
Do you know why Cambridge put the water *below* the arrow? U coconaden. -
Originally posted by Flying grenade:
photo of qn , 2004 p3 qn 3either bii)
https://www.dropbox.com/s/nzhs6tqnlys1r3s/20160928_124011.jpg?dl=0
for the 2011 buffer qn can find all the pH .
for this qn , is it possible to find pH of end point?? i dk the number of moles, as they didnt give initial vol of ethanoic acid, and hence i cannot find the no. of moles of ch3co2- and hence the conc of ch3co2- and hence cant find pH and end pt is it??
https://www.dropbox.com/s/ca7s5ylc59mgnwh/20160928_124018.jpg?dl=0
eh sorry. qn have give 20cm3. eyes didnt see properly just now
U coconaden. The information given in the TYS qn IS INDEED sufficient to calculate pH at end point. But did the qn ask u for pH at endpoint? -
the qn neither specified to find pH at end pt , nor didnt specify
the qn zz sia , say label end pt, so just draw a cross to label it, with the appropriate location ah? the requirement to label VNaOH and pH is ambiguous.
u tell me in doubt always find
anyway, i wanted to find cos i want to practice
is my method correct to find the pH and end pt from the photo above?
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Originally posted by UltimaOnline:
Do you know why Cambridge put the water *below* the arrow? U coconaden.eh why???
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oh yea and the qn must always give the conc and vol of the weak acid.
giving the conc of the strong base, to deduce how much vol of strong base is needed, the vol and conc of weak acid must be given
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lolol
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Originally posted by Flying grenade:
the qn neither specified to find pH at end pt , nor didnt specify
the qn zz sia , say label end pt, so just draw a cross to label it, with the appropriate location ah? the requirement to label VNaOH and pH is ambiguous.
u tell me in doubt always find
anyway, i wanted to find cos i want to practice
is my method correct to find the pH and end pt from the photo above?
Which photo? -
Originally posted by Flying grenade:
eh why???
Hint : Cambridge is indeed using anhydrous NaBH4, not NaBH4(aq). -
Originally posted by UltimaOnline:
Which photo?photo of qn , 2004 p3 qn 3either bii)
https://www.dropbox.com/s/nzhs6tqnlys1r3s/20160928_124011.jpg?dl=0
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Originally posted by Flying grenade:
photo of qn , 2004 p3 qn 3either bii)
https://www.dropbox.com/s/nzhs6tqnlys1r3s/20160928_124011.jpg?dl=0
I only see the TYS qn. U said ur photo contained ur working to solve for pH at endpoint? Or else how u expect me to check ur working? -
Originally posted by UltimaOnline:
Hint : Cambridge is indeed using anhydrous NaBH4, not NaBH4(aq).why 1. NaBD4 2. D2O
is it followed by D2O ah?
i dont get this thing ....
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Originally posted by UltimaOnline:
I only see the TYS qn. U said ur photo contained ur working to solve for pH at endpoint? Or else how u expect me to check ur working?ok i take photo show u later, now not with my bag
basically i use [oh-] = root(kb x [base])
then find pH from 14-pOH
[base] i use molCH3CH2COOH=molCH3CH2COO- divided by 20+10 = 30cm3
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Originally posted by Flying grenade:
why 1. NaBD4 2. D2O
is it followed by D2O ah?
i dont get this thing ....
Draw out the mechanisms for LiAlH4 on aldehyde, ketone, carboxylic acid, ester, amide, nitrile (which Cambridge *can *ask as a distinction A grade H2 Chem question) to properly understand why the reagents and conditions have to be written this way (ie. why 1st step must be anhydrous, why 2nd step must hydrolyze).If you cannot draw the mechanisms, go ask your school teacher or private tutor to teach you.
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Originally posted by Flying grenade:
ok i take photo show u later, now not with my bag
basically i use [oh-] = root(kb x [base])
then find pH from 14-pOH
[base] i use molCH3CH2COOH=molCH3CH2COO- divided by 20+10 = 30cm3
Correct.For a more challenging question, Cambridge can use a multiprotic or polyprotic acid instead, eg. amino acid, with Ka1, Ka2, Ka3, Kb1, Kb2, Kb3. Most Singapore JC H2 students get confused as to which K values to use for calculations.
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Originally posted by UltimaOnline:
Correct.For a more challenging question, Cambridge can use a multiprotic or polyprotic acid instead, eg. amino acid, with Ka1, Ka2, Ka3, Kb1, Kb2, Kb3. Most Singapore JC H2 students get confused as to which K values to use for calculations.
yes, 2011 was a qn involving diprotic acid
triprotic acid haven't come out *yet*
ok thanks for ur help
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ooh Na in ethanol can function as a reducing agent?
https://www.dropbox.com/s/2akyl0ommc62wvc/20160928_174233-1.jpg?dl=0
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Originally posted by Flying grenade:
ooh Na in ethanol can function as a reducing agent?
https://www.dropbox.com/s/2akyl0ommc62wvc/20160928_174233-1.jpg?dl=0
Of course (don't you know why?). Nonetheless, you're advised to stick to the different reducing reagents and conditions specified for the different reactions in the H2 syllabus. Often, Cambridge is testing you on whether you've correctly memorized the industry methods for a particular reaction, and may not accept alternative methods (even if they work). -
EHHH I KNOW WHY LIAO COS H2 PRODUCED ((((:
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why the need to write a metal and acid, whereby can just state H2(g) as the reducing agent? -__-
e.g. sn hcl , na ethanol
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oh ok la sn2+ can function as a reducing agent
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oh Na is also a reducing agent haha, most metals are haha
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Originally posted by Flying grenade:
EHHH I KNOW WHY LIAO COS H2 PRODUCED ((((:
NOT
It's actually because of this mechanism (wouldn't it be nice if Cambridge asked you to draw out this mechanism in the A level exam?)