ocr a f325 revision thread

when is the exam? morning or afternoon?

Ok so say you have 0.2 moles of HCl and 0.2 moles of NaOH
The equation is HCl+NaOH--->H2O + NaCl
So 0.2 moles of water is formed, you need to scale it up to form 1 mole of water.

So do 1/0.2 = 5 so you have to scale up the Q=mcT by 5 to form 1 mole

Hi guys my answer is 1:1 but I don't know if anyone can calculate anything different?
Ignore the my scribbles I had no idea at the time of what to do.

you're a lifesaver! thankyou so when don't you scale? because i dont normally i dont think!? sorry for all the questions!


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Does anyone find themselves overthinking/ over-complicating questions? I was doing a q just now and I couldn't figure out whether to use K or cetigrade in a Q=mct question, even though I've done this type of q a million times before :/ I always think ocr are trying to catch us out

It's ok This helps me revise too.

You always need to scale because by definition "The enthalphy change of neutralisation is the enthalpy change that occurs when an acid and a base react fully to form 1 mol of water"
So always scale to 1 mole

First work out the moles of KMnO4.
cv/1000=n
So 0.02*23.45/1000 = 4.69*10^-3
Look at the overall all equation, 5 moles of H2O2 reacts with 2 moles of MnO4-
So (5/2)*(4.69*10^-3)
=0.011725 moles
Now this is the mole of H2O2 in the DILUTED solution, it asks for undiluted so
You have to times moles by a factor of 10.
=0.11725 mole of H2O2

Now it wants it in gdm-3 not, molcm-3
So the mr of H2O2 is 34.
Now I am really not sure where they get the 40 from the markscheme from tbh.... maybe some one could help?

Does anyone find themselves overthinking/ over-complicating questions? I was doing a q just now and I couldn't figure out whether to use K or cetigrade in a Q=mct question, even though I've done this type of q a million times before :/ I always think ocr are trying to catch us out

You're nearly there just one more step..c=n/v, so divide ur number of moles by 25/1000 as that's the volume of h2o2..multiplying by 40 and dividing by 25/1000 is the same thing

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See that's what I don't get, we work out no. of moles for 250cm3 so wouldn't be 4?

Remember, Q=mcT is degrees
^G=^H-T^S is in kelvin

when working out buffers do the units have to be in moles or conc

bump

Because if equal volumes = 1+1, double of the existing volume being added would be 1 (existing) +2 (added volume)

Idk, can someone refute or verify this? I could just be complicating things, I probably am.

is this true? for the dilute thing that the question was asked

Thanks! I'm going to stay calm and try not to confuse myself during the exam -_-

Btw what is the unit for G?

In a neutral solution does [H+]=[OH-] ?