ocr a f325 revision thread

Guyyyyyss
does anyone have the jan 13 ms ???

I worked out how to do it, using kw again but I got the right answer this time. I dunno how I didn't get this answer first time, must have forgotten to log it or something.
So you're finding pH, [H+]= kw/[OH-]
[OH-]=100x[H+], kw=1.00x10^-14
so [H+]=1.00x10^-14/100[H+]
[H+]^2= 1.00x10^-14/100
[H+]= square root of 1.00x10^-14/100
Then you minus log the [H+], -log10(1x10-8
which gives 8

Using quadratic equation to find pH

Ka=[H+][A-]/[AH]
Ka=[H+]^2/[AH-H+]
Ka[AH-H+]=[H+]^2
[H+]^2-Ka[-H+]-Ka[AH]
Use quadratic
I doubt it will come up but you never know

Good, thanks I was getting worried. Also they put [H+] as 1.99x10^-9, yet I got 1.95x10^-5, same error?

Can someone explain buffers for me? i get really confused

Can someone explain buffers for me? i get really confused

with complex ions like [Ni (Cl)4] what is the bond angle and shape?

In my revision guide it says tetrahedral with angle of 109.5 degrees
but in my notes from class they say square planar with angle of 90.

Which is right?

Thanks in advance

ok so if anyone could help me with this I'd be grateful... I find it difficult to rearrange formulas, but I find it easy when they are put into the triangle formula format...how do I put the Gibbs Free energy equation into a triangle equation format? and what would ds be equal to ? thx in advance..

I am mainly referring to question 5d on the june 2011 paper..

anyone ??

o lets take the buffer for blood- which has to remain at the pH of 7.45
So its carbonic acid, H₂CO₃ dissociating into HCO_3^- and H⁺
H₂ CO₃ ⇄ HCO₃^- + H⁺

Definition for buffer solution: a solution (made from a weak acid and its conjugate base) that can minimise the change in pH when small amounts of alkali(OH^-) and acid (H⁺) is added

so when OH^- added this reacts with the H⁺ in the blood it makes H₂O. The equilibrium shifts to the right to make more H⁺'s, cos some have been lost.

when H⁺ is added, it reacts with HCO₃^- and makes more H₂CO₃. This shifts the equilibrium to the left. So all this ultimately controls the pH.

Hoped that all helped

What triangle equation.

http://www.ocr.org.uk/Images/81033-unit-f325-equilibria-energetics-and-elements-specimen.pdf

On question 2d, does anybody else get 0.013, instead of 0.13?

Write an equation and you'll be able to see it. Every mole of H2SO4 reacts with a monobasic base to form 2 moles of water, so twice the enthalpy change. The salt ions are just spectator ions.



well yes, read the definition of standard enthalpy change of neutralisation. It's all about the one mole of H2O, so you have to divide by how many moles of water you've formed.

It's tetrahedral.

I mean you know how n=m/mr is represented in a triangle in the text book, how do I do that for the gibbs free energy equation?

From my notes it tends to be only Ni/Pt metal ion complexes which take the square planar shape with 90 degree bond angles. The rest such as Co or Cu as central metal ion use tetrahedral with 109.5 bond angles.