Rishi's Chemistry Thread

[Rishi97]

Oil plus sodium hydroxide => fatty acid (well, conjugate base of fatty acid at least) + glycerol
Then plus methanol => methyl ester
You have biodiesel and glycerol as your two layers (one is non-polar, one is polar)

Each school's SACs will be different. Mine was on aspirin, spectroscopy, biomolecules, naming things and reaction pathways from memory. It wasn't a hard SAC but I completely messed up one of the IR questions (teacher put an amine in the SAC and I hadn't looked over what those absorptions looked like -.-)

Thanks for that
at least, you still got a 50

[Rishi97]

In regards to organic chem, what is meant by cracking?

[Yacoubb]

In regards to organic chem, what is meant by cracking?

The reaction by which an alkane is converted to an alkene and an alkane.


Example:

C8H18 ---> C4H8 + C4H10
Octane ---> Butene + Butane

[Rishi97]

Lactic acid (HC₃H₅O₃) is a product of anaerobic respiration in muscle cells. It accumulates in muscle tissue during exertion, causing pain. In a 0.15M solution of lactic acid, the acid is 3.0% ionised.
i) Determine the concentration of the C₃H₅O₃^- anion in a 0.15M aqueous solution of lactic acid

Thanks heaps

[lzxnl]

3.0% ionised => concentration of ionised lactate = 3% * 0.15 M = 3*10^-2 * 1.5*10^-1 M = 4.5*10^-3 M

[Rishi97]

A 10mL solution of HCl has a pH of 2. WHat volume of water,in mL, must be added to it to change the pH to 3?
a) 10
b) 90
c) 99
d) 100

Thanks

[brightsky]

Need to dilute by a factor of 10, i.e. turn 10 mL into 100 mL. So need to add 90 mL. B.

[Yacoubb]

A 10mL solution of HCl has a pH of 2. WHat volume of water,in mL, must be added to it to change the pH to 3?
a) 10
b) 90
c) 99
d) 100

Thanks

This question ultimately requires you to find the concentration first.

At a pH of 2:

pH = -log10[H3O+]
2 = -log10[H3O+]
[H3O+] = 10^-2
[H3O+] = 0.1 M

At a pH of 3:

pH = -log10[H3O+]
3 = -log10[H3O+]
[H3O+] = 10^-3
[H3O+] = 0.01 M

Now, we can use:

C1V1 = C2V2
0.1M * 10mL = 0.01M * V2
V2 = 100mL

Back to the stem of the question, it asks you to find the amount of water needed to change the pH to 3. 100mL - 10mL = 90mL. Hence, the answer is B.

[Rishi97]

This question ultimately requires you to find the concentration first.

At a pH of 2:

pH = -log10[H3O+]
2 = -log10[H3O+]
[H3O+] = 10^-2
[H3O+] = 0.1 M

At a pH of 3:

pH = -log10[H3O+]
3 = -log10[H3O+]
[H3O+] = 10^-3
[H3O+] = 0.01 M

Now, we can use:

C1V1 = C2V2
0.1M * 10mL = 0.01M * V2
V2 = 100mL

Back to the stem of the question, it asks you to find the amount of water needed to change the pH to 3. 100mL - 10mL = 90mL. Hence, the answer is B.

Thanks for that Yacoubb

[Rishi97]

Would different low resolution proton NMR spectra be observed for C₃H₆O₂ (l) as opposed for C₃H₆O₂ (aq) ? Give a reason for your answer

Many thanks :-)

[Rishi97]

Citric acid, C₆H₁₂O₆, the main acid in lemon juice, is a tripotic acid. When 20.00ml aliquots of lemon juice were fully titrated with 0.350M NaOH, the volume of the average titre was 37.09mL. What was the concentration, in g/L, of citric acid in the lemon juice?
a) 0.831
b) 2.50
c) 41.5
d) 374

Thanks

[Rod]

Hey Rishi

Assuming it's this reaction 3NaOH + C6H8O7 → NaH2C6H5O7 +3 H2O

n(NaOH) = 0.350 X 0.03709 = 0.01298 MOL
n (citric acid) = 1/3 x 0.00432 mol
c (Citric acid) + 0.00432 /0.02 = 0.216 mols per litre
c (citric acid) = 0.216 x 192 = 41.5

= C

Assuming it's the reaction I have given you.

[Rishi97]

Hey Rishi

Assuming it's this reaction 3NaOH + C6H8O7 → NaH2C6H5O7 +3 H2O

n(NaOH) = 0.350 X 0.03709 = 0.01298 MOL
n (citric acid) = 1/3 x 0.00432 mol
c (Citric acid) + 0.00432 /0.02 = 0.216 mols per litre
c (citric acid) = 0.216 x 192 = 41.5

= C

Assuming it's the reaction I have given you.

Hey Rod
So where did you get that formula from? Did you write out each step of the ionisation?
Thanks

[Rod]

Hey Rod
So where did you get that formula from? Did you write out each step of the ionisation?
Thanks

Nah,

It's just an acid-base reaction

Citric acid is the acid, so donates one proton, while NaOH accepts one proton and becomes the water. Citric acid on the other hand loses on proton.

hope that helps

[Rishi97]

Stupid question, but where did this come from? C6H8O7