The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

ocr a f325 revision thread

Scroll to see replies

Original post by Joey952
when working out buffers do the units have to be in moles or conc


I was wondering about this too but I think as long as you have the salt AND acid in moles or concentration it'll work?
Original post by alltimeemilyxo
% yield is actual over theoretical isnt it?
so you just do moles of sodium ferrate over moles of iron oxide assuming they are 1 to 1 ratio and times by 100


i don't get the right anser..
Original post by Sarangtaec
I was wondering about this too but I think as long as you have the salt AND acid in moles or concentration it'll work?


BOTH have to be in conc, that's what the [...] represents.
Reply 3103
Avatar for Vhai
Vhai
Can anyone explain june 10 question 7b?
Reply 3104
Avatar for kuku2013
kuku2013
morning all
Original post by georgiaaaxo
if you get an enthalpy of neutralisation question, and there are different volumes of acid and alkali, in the Q equation do you put the volume of acid reacted + the volume of alkali as m?
and what else would change? what would you divide by in Q/moles if no. of moles of acid and alkali are different???


Posted from TSR Mobile


Q=mcT
m is the mass, you use the volume of a solution because the mass of water is roughly 1gcm-3

So if you add 20 cm3 of HCl to 40 cm3 of NaOH, m=60 cm3

With the moles you use the smallest mole because the excess acid or base has not reacted and not formed any H2O

Hope this makes sense :smile:
Original post by rival_
Apparently the undiluted solution is a reference to the fact that in your volumetric you've got only water and the 25cm^3 of the (can't remember exactly what it was lol) so basically you multiply by 40 to scale it up to 1dm^3.


why 40? ;s
Reply 3107
Avatar for kuku2013
kuku2013
hey guys do we need to learn the colours on pg 205
Original post by master y
why 40? ;s


Multiplying by 40 is the same as dividing by 25/1000 (25cm3 is the volume but u want it in dm3)
Good luck everyone! I have a general studies exam too T_T
Original post by DudeBoy
Q=mcT
m is the mass, you use the volume of a solution because the mass of water is roughly 1gcm-3

So if you add 20 cm3 of HCl to 40 cm3 of NaOH, m=60 cm3

With the moles you use the smallest mole because the excess acid or base has not reacted and not formed any H2O

Hope this makes sense :smile:


thankyou SO much!!!!! always got confused when things are in excess - thanks :biggrin:


Posted from TSR Mobile
Original post by DudeBoy
BOTH have to be in conc, that's what the [...] represents.


Really??? I swear I did it in moles once and it worked =\ Oh well, ok thanks haha
Original post by Vhai
Can anyone explain june 10 question 7b?


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?
Reply 3113
Avatar for kuku2013
kuku2013
if a transition metal does not have a partially filled d-orbital it forms white coloured compounds. is this correct?
Original post by DudeBoy
Q=mcT
m is the mass, you use the volume of a solution because the mass of water is roughly 1gcm-3

So if you add 20 cm3 of HCl to 40 cm3 of NaOH, m=60 cm3

With the moles you use the smallest mole because the excess acid or base has not reacted and not formed any H2O

Hope this makes sense :smile:


oh just one more question - when it says in the textbook, scale to make sure 1 mole of h2o is formed, what does that mean?! the last 2 steps in the textbook I don't really get, and I swear we don't do them when we are doing the enthalpy of neut? all about scaling for 1 mole of h2o :/


Posted from TSR Mobile
Original post by Sarangtaec
Really??? I swear I did it in moles once and it worked =\ Oh well, ok thanks haha


Hm, maybe the it was the same, if you have 4moles and pressurize it by 1dm3 your conc is 4moldm-3, so maybe you got lucky :smile:
Reply 3116
Avatar for rival_
rival_
10
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.
IMG_20130612_073630.JPG120.3KB
when is the exam? morning or afternoon?
Reply 3118
Avatar for AyshaK
AyshaK
Original post by Joey952
what is the equation when OH is the subject of your formula ?


Are you reffering to Kw=[H+][OH-] ?
Original post by georgiaaaxo
oh just one more question - when it says in the textbook, scale to make sure 1 mole of h2o is formed, what does that mean?! the last 2 steps in the textbook I don't really get, and I swear we don't do them when we are doing the enthalpy of neut? all about scaling for 1 mole of h2o :/


Posted from TSR Mobile

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

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you