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

1. Hey does anyone know if the PhillipAllan F325 revision guide is worth getting??
2. (Original post by sportycricketer)
Right ok so lets look at the key facts here:

5.119 = CO2 1.575 = H2O

You know that CO2 Mr = 44 and in it C is 12

so you can work out a fraction of C in 5.119 of CO2 which is 12/44 x 5.119 = 1.396

Same with H2O = 18 and H is 2

2/18 x 1.575 = 0.175

Now you have the g of H2 and C, so you can subtract that from the total to find O2

4.362 - (1.396 + 0.175) = 2.791

Now you can work out the empirical firmula

C: H: O
1.396: 0.175: 2.791

divide through by their Mr

C 1.396/12 = 0.1163
H 0.175/1 = 0.175
O 2.791/16 = 0.1744

0.1163:0.175:0.1744
simplifying it

1: 1.5: 1.5
to get a whole number ratio you can multiply by 2 to get
2: 3: 3

This gives you C2H3O3 to get 75

150/75 = 2

2 x (C2H3O3) = C4H6O6

Hope this helps
Slightly off the topic..
Do I need to know why chloride ligands are larger than water ligands!
ANd also about stronger repulsion I am guessing its repulsion between chloride ions
3. (Original post by student777)

"Compound E has the formula CxHyOz. 4.362g of E was burned in oxygen to form 5.119g of CO2 and 1.575g H2O. Mass spec showed a peak at 150.

Calculate the molecular formula of E."

I had a look at the mark scheme but I can't follow what it's doing :s Do you think we'll get something similar in our paper?
There is a question similar to this in specimen paper..Uve got time to learn it
4. Ive been told that we wont be asked about explaining differences in ligands colour !

There is like 10 mark question in 2007 ..but apparently it was taken out from syllabus some time later
5. Has anyone got any advice for reaction mechanisms generally? I never seem to be able to work them out...I can see why it would work when I look at the answer but...well that doesn't help in an exam.
Am I right with the following statements?
The rate determining step is the slowest step in the reaction mechanism
If a reactant appears in the rate equation, that reactant is involved in the rate determining step. The order of the reactant tells you how many of the particles of the reactant are involved in the rate determining step.
The overall (stoichometric) equation tells you how many particles of the reactants are involved in total
Does anyone have a pack of questions/worksheet or anything with these types of questions? I could really do with specific practice on these

Oh and, anyone found any examples of calculating the pH with a di/tri protic acid or base? (other than pg 159 8aiii), could really do with practice/help on those too.
6. hi to everybody. i was just wondering how much revision everyone has done for this exam so far, as i have also got the biology exam which is like 2 days before the chemistry exam and i am finding it quite hard to fit everything in.
7. (Original post by inam_ayaz)
hi to everybody. i was just wondering how much revision everyone has done for this exam so far, as i have also got the biology exam which is like 2 days before the chemistry exam and i am finding it quite hard to fit everything in.
Me and many people on here included are also doing F215 two days before! It's quite a lot to fit in tbh but I guess you can dedicate one day for each subject. That's what I did earlier this week.. spent tuesday doing chem and wednesday doing biology
8. Hey, I'm finding it hard to get my head around buffers. It is composed of a weak acid, the conjugate base and the salt of the weak acid. How does the salt of the weak acid form? On adding acid, does the conjugate base react to form the weak acid? I revise over it again and again, I guess I just don't understand it properly. Could someone explain it to me please?
9. (Original post by arvin_infinity)
Ive been told that we wont be asked about explaining differences in ligands colour !

There is like 10 mark question in 2007 ..but apparently it was taken out from syllabus some time later
Yep thats right we don't need to know about that its not on our spec (luckily!)
10. can any1 help explain why F325 is making me feel suicidal??
11. (Original post by INeedToRevise)
Hey, I'm finding it hard to get my head around buffers. It is composed of a weak acid, the conjugate base and the salt of the weak acid. How does the salt of the weak acid form? On adding acid, does the conjugate base react to form the weak acid? I revise over it again and again, I guess I just don't understand it properly. Could someone explain it to me please?
Ok, here goes. You seem a bit confused on how a buffer forms, so here it is:

You make a buffer solution by e.g. Adding NaOH to an EXCESS of CH3COOH. Therefore you get CH3COOH and some CH3COONa, which is the salt of the weak acid. Some CH3COOH remains.

The weak acid partially dissociates but the ionic salt fully dissociates. So from the salt we get large amounts of CH3COO- and Na+. For the purposes of the buffer solution we can ignore the Na+.

An equilibrium is set up: CH3COOH <-> CH3COO- + H+
(that should be an equilibrium arrow above)

There are large amounts of the undissociated acid, because it is a weak acid, and large amounts of the conjugate base, because the salt fully dissociates. There is only a small amount of H+ because it comes from the weak acid, which only partially dissociates.

Are we alright so far? Let me know if you need more help
12. The 10 marker on transition metals jan '08, are we meant to know all about vanadium and its reactions?is it on our specification? thanks
13. (Original post by student777)
Ok, here goes. You seem a bit confused on how a buffer forms, so here it is:

You make a buffer solution by e.g. Adding NaOH to an EXCESS of CH3COOH. Therefore you get CH3COOH and some CH3COONa, which is the salt of the weak acid. Some CH3COOH remains.

The weak acid partially dissociates but the ionic salt fully dissociates. So from the salt we get large amounts of CH3COO- and Na+. For the purposes of the buffer solution we can ignore the Na+.

An equilibrium is set up: CH3COOH <-> CH3COO- + H+
(that should be an equilibrium arrow above)

There are large amounts of the undissociated acid, because it is a weak acid, and large amounts of the conjugate base, because the salt fully dissociates. There is only a small amount of H+ because it comes from the weak acid, which only partially dissociates.

Are we alright so far? Let me know if you need more help
Thanks That has helped me. So what happens when you add acid/alkali?
And do we need to know the equation to make a buffer solution, so like the equation to react NaOH with excess CH3COOH?
14. There is a question in jan 2007 paper..
says what colour change occurs?
in cr207 2- + h20 <--> 2 cro4 2- + 2h+

Am not sure if its in the textbook
15. (Original post by INeedToRevise)
Thanks That has helped me. So what happens when you add acid/alkali?
And do we need to know the equation to make a buffer solution, so like the equation to react NaOH with excess CH3COOH?
Lets see how much I remember..Hmmm

It combines to H+ ...>forms water

Equil. shifts to right ( cuz u removed H+) to minimise the change---> more H+ produced

so minimises ph changes

I let u guess the other one
16. (Original post by arvin_infinity)
Lets see how much I remember..Hmmm

It combines to H+ ...>forms water

Equil. shifts to right ( cuz u removed H+) to minimise the change---> more H+ produced

so minimises ph changes

I let u guess the other one
So if you add acid, the conjugate base reacts with H+ to form the weak acid. The equilibrium shifts to the left to remove the extra H+.

Hope this comes up now. Haha
17. (Original post by INeedToRevise)
So if you add acid, the conjugate base reacts with H+ to form the weak acid. The equilibrium shifts to the left to remove the extra H+.

Hope this comes up now. Haha
Thats right

Am slightly worried about colour of transition metals:

WOuld be epic if someone list them all here

1. Cro4 2- Yellow
2.cucl2- Pale blue
..
18. (Original post by INeedToRevise)
Thanks That has helped me. So what happens when you add acid/alkali?
And do we need to know the equation to make a buffer solution, so like the equation to react NaOH with excess CH3COOH?
You don't need to know any specific equations, but they may ask you to apply general principles. For example 'A buffer solution was made by reacting ammonia and HCOOH. Suggest the equation'. It's just acid base reactions really.
19. (Original post by student777)
You don't need to know any specific equations, but they may ask you to apply general principles. For example 'A buffer solution was made by reacting ammonia and HCOOH. Suggest the equation'. It's just acid base reactions really.
Ok.
20. (Original post by arvin_infinity)
Thats right

Am slightly worried about colour of transition metals:

WOuld be epic if someone list them all here

1. Cro4 2- Yellow
2.cucl2- Pale blue
..
Do we need to know the colour of chromium compounds?

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