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Equilibria, Energetics and Elements (F325) - June 2011 Exam.

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Reply 540
Avatar for Rosi M
Rosi M
11
Original post by INeedToRevise
Yeah, I have psychology on the 21st as well, thats going to be cramming as well except I have biology the next day :cry2: so the evening of the 21st is definitely gonna be cramming for biology. After that its all chemistry.
Thats good :smile: I should do that. I quite like benzene haha


Ooo you probably have the same psychology exam as me.. PSYA4? Lets not even talk about that! :eek:

I love benzene too but dont think it loves me back one bit!!!!
Reply 541
Avatar for M_I
M_I
How do you find the pH of a dibasic acid?

Thanks.

Rep awaiting.
Original post by M_I
How do you find the pH of a dibasic acid?

Thanks.

Rep awaiting.


Assuming that the acid fully dissociates then the concentration of H+ is double that of the acids, then you can just - log it to find the PH
Reply 543
Avatar for M_I
M_I
Original post by Jtking3000
Assuming that the acid fully dissociates then the concentration of H+ is double that of the acids, then you can just - log it to find the PH


So say H2SO4 (its is a strong acid right?) has a conc. of 0.25 mol dm-3, then

[2Acid] = [H+]

so [H+] = 0.5 mol dm-3

pH = - log(0.5) = 0.30

Yeah?






Also, would we be asked to find the pH of a weak base? If yes, how?

Thanks so much. :smile:

.
(edited 12 years ago)
Original post by M_I
So say H2SO4 (its is a strong acid right?) has a conc. of 0.25 mol dm-3, then

[2Acid] = [H+]

so [H+] = 0.5 mol dm-3

pH = - log(0.5) = 0.30

Yeah?

Also, would we be asked to find the pH of a weak base? If yes, how?

Thanks so much. :smile:


Yes thats correct, and calculating the PH of a weak base isn't on the syllabus (or in the textbook at least lol)
Reply 545
Avatar for M_I
M_I
Original post by Jtking3000
Yes thats correct, and calculating the PH of a weak base isn't on the syllabus (or in the textbook at least lol)


Thought so, just making sure.

Oh and do you know if we have to memorise all the colours? The ones on page 88 of the CGP book.
Original post by M_I
Thought so, just making sure.

Oh and do you know if we have to memorise all the colours? The ones on page 88 of the CGP book.


I don't have the CGP book, I think the colours we have to memorise are the precipitate colours to distinguish between Fe2+/3+ and the before/after ligand substitution reactions but I'm not 100% certain.
Original post by M_I
So say H2SO4 (its is a strong acid right?) has a conc. of 0.25 mol dm-3, then

[2Acid] = [H+]

so [H+] = 0.5 mol dm-3

pH = - log(0.5) = 0.30

Yeah?








Are you sure about that? the book has nothing on how to calculate ph differently in multiprotic acids. Ka decreases with each dissociation anyway so I don't think you can just double the concentration. Correct me if i'm wrong though lol
Reply 548
Avatar for M_I
M_I
Original post by Killmepls
Are you sure about that? the book has nothing on how to calculate ph differently in multiprotic acids. Ka decreases with each dissociation anyway so I don't think you can just double the concentration. Correct me if i'm wrong though lol


Ka is for weak acids.

Yeah, I don't think its in the book, but its says for monobasic acids [H+] = [Acid]
Original post by Killmepls
Are you sure about that? the book has nothing on how to calculate ph differently in multiprotic acids. Ka decreases with each dissociation anyway so I don't think you can just double the concentration. Correct me if i'm wrong though lol


I believe thats Ka does decrease but i think they'd ask you to assume that it completely dissociates as it would get slightly hairy otherwise. I've seen a question where they asked you to calculate the PH of Ca(OH)2 and they asked you to assume that it fully dissociates and you had to double the concentration of OH-.
Reply 550
Avatar for M_I
M_I
Original post by Jtking3000
I don't have the CGP book, I think the colours we have to memorise are the precipitate colours to distinguish between Fe2+/3+ and the before/after ligand substitution reactions but I'm not 100% certain.


Ok, we probs don't need to memorise them.
Original post by M_I
So say H2SO4 (its is a strong acid right?) has a conc. of 0.25 mol dm-3, then

[2Acid] = [H+]

so [H+] = 0.5 mol dm-3

pH = - log(0.5) = 0.30

Yeah?






Also, would we be asked to find the pH of a weak base? If yes, how?

Thanks so much. :smile:

.


WE DONT NEED TO KNOW HOW TO WORK THE PH OF MONOBASIC ACIDS OR WEAK BASES because it clearly says in the spec that "Students should be able to work out the pH of: strong bases, strong monobasic acids and weak acids!
Original post by Jtking3000
I believe thats Ka does decrease but i think they'd ask you to assume that it completely dissociates as it would get slightly hairy otherwise. I've seen a question where they asked you to calculate the PH of Ca(OH)2 and they asked you to assume that it fully dissociates and you had to double the concentration of OH-.


:O thanks, yeah i'll look out for that. Just checked the text book they have no examples of compounds which release two OH- or even two H+ but this is ocr you never know :P
Reply 553
Avatar for M_I
M_I
Ok, it seems like we don't need to know it, but just incase lol.

What pH value does a strong acid titration curve start from? Is it 2?

And what about weak acid? Around 4?

And what about where alkali's end?

Is strong like 14?
Weak, 10?
Original post by M_I
Ok, it seems like we don't need to know it, but just incase lol.

What pH value does a strong acid titration curve start from? Is it 2?

And what about weak acid? Around 4?

And what about where alkali's end?

Is strong like 14?
Weak, 10?


If they give you data, you might have to calculate it! :smile:
Reply 555
Avatar for M_I
M_I
Original post by haydyb123
If they give you data, you might have to calculate it! :smile:


I didn't know that. :eek:

Do you have an example question?
Original post by M_I
I didn't know that. :eek:

Do you have an example question?


No sorry, but it shouldn't be different to what we normally do, except we'll plot our answers on a graph!
Kinetics
Equilibria
Acids and Bases
Redox reactions
Electrode Potentials and Fuel Cells
Enthalpy, Entropy and Free Energy
Transition metals

Probably a question on each topic..

Success though will not come from purely learning all the specification content and memorising mark schemes. Although this will help, application will also be tested using questions set in novel contexts

Does this mean the Trends and Patterns, Transiton Elements and Unifying Concepts old spec papers are not relevant or worthwhile practicing?
Knowledge of the following is expected for F325...in AS and some of F324 as well...


AS Unit F321: Atoms, Bonds and Groups
1.1.3 Acids;
1.1.4 Redox.
1.3.2 Group 2 (acid reactions with metals, carbonates and bases).

AS Unit F322: Chains, Energy and Resources
2.3.1 Enthalpy Changes;
2.3.2 Rates and Equilibrium.
2.4.1 Chemistry of the Air.
Original post by intellectual1
Kinetics
Equilibria
Acids and Bases
Redox reactions
Electrode Potentials and Fuel Cells
Enthalpy, Entropy and Free Energy
Transition metals

Probably a question on each topic..

Success though will not come from purely learning all the specification content and memorising mark schemes. Although this will help, application will also be tested using questions set in novel contexts

Does this mean the Trends and Patterns, Transiton Elements and Unifying Concepts old spec papers are not relevant or worthwhile practicing?


I wouldn't say they weren't worth practising... I found the UC papers particularly helpful because I tend to make stupid mistakes in calculations. So I would say if you need help on specific topics then you should do the old spec papers. However if you want an idea of the question styles etc then they're not going to be too helpful.

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