ocr a f325 revision thread

Hi could anyone explain how to complete question 2(d) of the specimen paper??

You know the value of Ka (1.3x10^-10) and you can work out the [H+] concentration from the pH of 8.71 by doing 10^-8.71, and you also know the value of [HA] as 0.200.

Use the equation [H+] = Ka x ([HA] \ [A-]) and rearrange it to get the conjugate base on it's own. Feed the numbers in and you should get the answer.

Rearranged it gives [C6H5O-] = (1.31x10^-10) x 0.200) all divided by the [H+] concentration.

Hope that helps.

Reply 2241

MathsNerd1

17

Original post by Jessie.xx

CAN SOMEONE HELP ME PLEASE!!!!

Calculate x in the formula FeSO4.xH2O from the following data: 24.4 g of iron (II)
sulphate crystals were made up to 1 dm3 of aqueous solution acidified with sulphuric acid. 25.0 cm3 of the solution required 16.6 cm3 of 0.022M K2Cr2O7 for complete reaction.

the answer is x=7 but I don't know where to start!!

Where did you find this?

Reply 2242

Pride

21

Original post by Jessie.xx

CAN SOMEONE HELP ME PLEASE!!!!

Calculate x in the formula FeSO4.xH2O from the following data: 24.4 g of iron (II)
sulphate crystals were made up to 1 dm3 of aqueous solution acidified with sulphuric acid. 25.0 cm3 of the solution required 16.6 cm3 of 0.022M K2Cr2O7 for complete reaction.

the answer is x=7 but I don't know where to start!!

You do what you always do, find number of moles in the titre.

But then you need an equation for the reaction to be able to do anything from there.

Reply 2243

slacky123

Original post by Pride

You do what you always do, find number of moles in the titre.

But then you need an equation for the reaction to be able to do anything from there.

We don't have to know the half equation for potassium dichromate do we?

Reply 2244

Better

8

Just did June 2012 again. Still finished in 1hr 50 mins, but it was no where near on the same level as January 2013.

I really hope the paper tomorrow is similar to Jan 13.

Reply 2245

otrivine

14

Original post by Pride

You do what you always do, find number of moles in the titre.

But then you need an equation for the reaction to be able to do anything from there.

so we get a half equation between Fe and dichromate, and use mole relations?

Reply 2246

Jessie.xx

Original post by MathsNerd1

Where did you find this?

http://www.a-levelchemistry.co.uk/AQA%20A2%20Chemistry/Unit%205/5.4%20Transition%20Metals/5.4%20Transition%20metals%20home.htm

They are AQA questions

Reply 2247

_sparks

11

Anyone know this?

Original post by _sparks

Are all alkalis strong bases, because they dissociate in water fully?

Also, why is the enthalpy change of neutralisation less exothermic for a weak acid?

Reply 2248

Namod

10

Original post by Jessie.xx

CAN SOMEONE HELP ME PLEASE!!!!

Calculate x in the formula FeSO4.xH2O from the following data: 24.4 g of iron (II)
sulphate crystals were made up to 1 dm3 of aqueous solution acidified with sulphuric acid. 25.0 cm3 of the solution required 16.6 cm3 of 0.022M K2Cr2O7 for complete reaction.

the answer is x=7 but I don't know where to start!!

recheck your question.

Reply 2249

Rumzz

Anyone done the Specimen paper? I really don't get Q2. First of I keep getting pH as 4.6 instead of 5.6 in 2c (ii) anyone care to show their working out?

Also carrying on, in question 2d) how do you get that answer. I don't think I really understood the method properly but Ka given as 1.3x10^-10moldm^-3
The rest of the info is here. Writing is red is meant to be the correct answer & method but tbh I just don't get any of it :/
Help?!

Reply 2250

otrivine

14

Original post by Namod

recheck your question.

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

may I ask question 1)A) i) FOR C2H4 can I compare experiments 1 and 3 or does it have to be 2 and 3?

Reply 2251

otrivine

14

Original post by Rumzz

Anyone done the Specimen paper? I really don't get Q2. First of I keep getting pH as 4.6 instead of 5.6 in 2c (ii) anyone care to show their working out?

Also carrying on, in question 2d) how do you get that answer. I don't think I really understood the method properly but Ka given as 1.3x10^-10moldm^-3
The rest of the info is here. Writing is red is meant to be the correct answer & method but tbh I just don't get any of it :/
Help?!

thats what I got, the mark scheme made a mistake

Reply 2252

MedMed12

21

where can I get the specimen paper? :smile:

Reply 2253

Jessie.xx

Original post by Pride

You do what you always do, find number of moles in the titre.

But then you need an equation for the reaction to be able to do anything from there.

Yes, I did that, I stopped when I got the number of moles of FeS04 IN 1000cm3, not sure how to carry on.

Reply 2254

MedMed12

21

Original post by Better

Just did June 2012 again. Still finished in 1hr 50 mins, but it was no where near on the same level as January 2013.

I really hope the paper tomorrow is similar to Jan 13.

Nooooooooooo!! Anything but Jan

Reply 2255

Gold Phoenix

Hey,

I'm confused about what takes place at each electrode (cathode and anode)...since the info I get from two chemistry books seem to contradict each other! So in need of some confirmation...

One of the reactions which takes place is the splitting of Hydrogen gas into its ions:

H2(g) -> 2H+ + 2e-

But where does this occur? CATHODE or ANODE?

Thanks in advance

P

Reply 2256

slacky123

Original post by _sparks

Anyone know this?

Also, why is the enthalpy change of neutralisation less exothermic for a weak acid?

Because weak acids have to break the O---H bond when the proton dissociates which is a slightly endothermic reaction meaning the overall reaction is less exothermic.

Posted from TSR Mobile

Reply 2257

Sarangtaec

Anyone remember what expression we INCLUDE H2O in?? I can't remember =\ I thought it was Kc?

Reply 2258

MathsNerd1

17

Original post by Better

Just did June 2012 again. Still finished in 1hr 50 mins, but it was no where near on the same level as January 2013.

I really hope the paper tomorrow is similar to Jan 13.

I'd rather prefer one that is like June 2012 as it would mean I'd be scoring in the 90's :tongue:

Reply 2259

Namod

10

Original post by Jessie.xx

CAN SOMEONE HELP ME PLEASE!!!!

Calculate x in the formula FeSO4.xH2O from the following data: 24.4 g of iron (II)
sulphate crystals were made up to 1 dm3 of aqueous solution acidified with sulphuric acid. 25.0 cm3 of the solution required 16.6 cm3 of 0.022M K2Cr2O7 for complete reaction.

the answer is x=7 but I don't know where to start!!

I got 10, where you got this question from?

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