Chemistry 3/4 2013 Thread

[Stick]

I keep forgetting the general formula for saturated, monounsaturated and polyunsaturated fatty acids. Is there a way I could work it out, rather than memorise formulae? Or is there an easier formula? I think my teacher taught me the format (I'm not sure if that formula is even correct, by the way, or what it even represents ).

[lzxnl]

A saturated fatty acid will be of the form Cn H2n O2
Your formula isn't quite correct. Think basic saturated acid: ethanoic acid. C2H4O2. Then add a CH2 group to get C3H6O2, propanoic acid. Etc.

[zvezda]

I keep forgetting the general formula for saturated, monounsaturated and polyunsaturated fatty acids. Is there a way I could work it out, rather than memorise formulae? Or is there an easier formula? I think my teacher taught me the format (I'm not sure if that formula is even correct, by the way, or what it even represents ).

That formula you have there is for mono-unsaturated

[Edward21]

I keep forgetting the general formula for saturated, monounsaturated and polyunsaturated fatty acids. Is there a way I could work it out, rather than memorise formulae? Or is there an easier formula? I think my teacher taught me the format (I'm not sure if that formula is even correct, by the way, or what it even represents ).

For me, the easiest way is if it's in the form -COOH, then work from there. Multiply the C by 2, if you need to add 1 to get the #H, then it's completely saturated. If you need to minus 1 to reach the #H then it's monounsaturated (1 double bond), if the H is less than that still then minus 2 off this time (aka what would've been a saturated CH2 region) and each time you do this it's another double bond.

EXAMPLE. Linoleic acid (this is a favourite that I've seen a few times) C17 H31 COOH. Follow what I said, 17*2=34 yet there's 31? So minus off one = 33 and you know it's unsaturated, monounsaturated at this point (1-double bond)! Then minus of another 2 to get 31, another double bond giving a polyunsaturated fat with 2 double bonds. I know it sounds sort of complicated at first, but if you can work it out for yourself, it really makes sense, rather than rote learning a formula which gives you an answer without comprehensive understanding.

[Stick]

^ That's what I was after. Thank you.

[Edward21]

With the 2nd question (I got the first), I have no idea on how to approach it and the VCAA solutions are really, well, blunt and don't really explain much. How novel that they mention this was probably the hardest question on the 2007 paper 

[lzxnl]

Assuming that the volume doesn't change due to the addition of HCl, pH of 2 means that the concentration of H+ would be 0.01M. In a one litre solution, this means 0.01 moles. Now, the solution of KOH initially had a concentration of OH- as 0.01M, so 0.01 moles of OH-. First, if we add 0.01 moles of HCl, we'll have a neutral solution as HCl is a strong acid. We need another 0.01 moles, so we need to add 0.02 moles of HCl in total.

[saba.ay]

How would I do the following questions from 2013 TSSM Unit 3 practice exam:

1) Name two functional groups present in lipid molecules.

Esters & ______________

2) The concentration of a hypochlorous acid solution, HOCl, is to be determined by titration with 0.100M sodium hydroxide. An appropriate indicator to use for the titration is:

a) methyl red
b) methyl orange
c) bromocresol green
d) phenolpthalein


Also, is the following an adequate definition of a primary standard:
A substance of such high purity that it's amount, in mol, can be calculated directly from it's mass.

Please and thanks in advance.

[Edward21]

How would I do the following questions from 2013 TSSM Unit 3 practice exam:

1) Name two functional groups present in lipid molecules.

Esters & ______________

2) The concentration of a hypochlorous acid solution, HOCl, is to be determined by titration with 0.100M sodium hydroxide. An appropriate indicator to use for the titration is:

a) methyl red
b) methyl orange
c) bromocresol green
d) phenolpthalein


Also, is the following an adequate definition of a primary standard:
A substance of such high purity that it's amount, in mol, can be calculated directly from it's mass.

Please and thanks in advance.

OMG, I know right 1) what the hell? I was like ummm, alkane, alkene is unsaturated??? 
2) I reckon phenolpthalein just because in the data book the HOCl is listed under weak acids, so the titration curve would start at pH 3-4, then go up to reach the 13ish area due to strong NaOH base, this means the equivalence point is >7 so phenolpthalein.

For the primary standard I went more VCE generic; known formula, pure, can make up standard solution of accurately known concentration

[Edward21]

Assuming that the volume doesn't change due to the addition of HCl, pH of 2 means that the concentration of H+ would be 0.01M. In a one litre solution, this means 0.01 moles. Now, the solution of KOH initially had a concentration of OH- as 0.01M, so 0.01 moles of OH-. First, if we add 0.01 moles of HCl, we'll have a neutral solution as HCl is a strong acid. We need another 0.01 moles, so we need to add 0.02 moles of HCl in total.

Oh I see, I've never encountered something of this nature, better late then never so I gotta neutralise the existing OH- ions 0.01mol, to form a neutral solution of pH 7 (strong acid with strong base?) then the H+ conc. would be 10^-7 yeah? How do I get from there to the pH of 2, or H+ conc. of 10^-2M = 0.01mol in this situation. I just can't find the link after the neutralisation part, that would give a solution of pH 7 with equal [OH-]=[H3O+]

[saba.ay]

2) I reckon phenolpthalein just because in the data book the HOCl is listed under weak acids...

Stupid question, but where exactly in the data book is that? I didn't know whether it was weak or strong, hence guessed. And yes, phenolpthalein is the correct answer. Thanks

For the primary standard I went more VCE generic; known formula, pure, can make up standard solution of accurately known concentration

Aren't those the properties of a primary standard- not the definition of what a primary standard is? :/

[Edward21]

Stupid question, but where exactly in the data book is that? I didn't know whether it was weak or strong, hence guessed. And yes, phenolpthalein is the correct answer. Thanks

Aren't those the properties of a primary standard- not the definition of what a primary standard is? :/

The acids "some weak acids" have the Ka values on the last page below the indicators, why the hell was that other indicator not even in our data book present in that MC???

And I just shove in key terms to make my point more valid haha, the point where it's really defined is where if can be used to make a solution of accurately known concentration, this links in with what you said about no. of mol in the same way

[lzxnl]

Oh I see, I've never encountered something of this nature, better late then never so I gotta neutralise the existing OH- ions 0.01mol, to form a neutral solution of pH 7 (strong acid with strong base?) then the H+ conc. would be 10^-7 yeah? How do I get from there to the pH of 2, or H+ conc. of 10^-2M = 0.01mol in this situation. I just can't find the link after the neutralisation part, that would give a solution of pH 7 with equal [OH-]=[H3O+]

To go from pH=13 to pH=2, you go from having a large excess of OH- to a large excess of H+. You have a one litre solution with neutral pH. To bring it down to pH 2, you need an extra 0.01M H+ which is 0.01 mol HCl. Add this to the 0.01 mol HCl needed to neutralize the solution and you get 0.02 mol.

[Edward21]

To go from pH=13 to pH=2, you go from having a large excess of OH- to a large excess of H+. You have a one litre solution with neutral pH. To bring it down to pH 2, you need an extra 0.01M H+ which is 0.01 mol HCl. Add this to the 0.01 mol HCl needed to neutralize the solution and you get 0.02 mol.

Mmmmm yeah I get you, it's just I feel so.. stupid that I didn't even consider neutralising the OH- it just flew right past me; I'll never get one of these questions wrong again!!

[clıppy]

What would the semi-structural formula of glycerol, oleic acid and the fat they form be?
I understand doing the glycerol and oleic acid ones but when it comes to the formed tri-ester i have no idea which side it begins from or what is the side chain