OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD

Evaluative on the relative strength of weak acids - could anyone give any advice and tips please? Greatly appreciated :smile:

Reply 103

tcameron

19

Original post by AqsaMx

If a d block element isn't a transition element e.g Zn2+ and Sc3+ does it form a white precipitate as opposed to a coloured one?
Could anyone confirm this?
And also, is there a way to predict what colour a transition metal ion will turn or do we just learn them?

They would form colourless compounds because they aren't transition metals so do not possess the property of being able to form coloured compounds

Reply 104

Saywhatyoumean

10

Original post by tcameron

They would form colourless compounds because they aren't transition metals so do not possess the property of being able to form coloured compounds

I thought we needed to say white compounds but colourless solutions?

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Reply 105

Can someone help me out with this question? Its from f325 june 10, how do we know the Mr of compound C?
I dont get why we multiply the mass with 24?

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Reply 106

Original post by AqsaMx

I have a qualitative on identifying transition metals through their reactions
Could anyone possibly give me advice for it ? :smile:

Anyone done this that could give tips?

Reply 107

Original post by AqsaMx

Anyone done this that could give tips?

I think you'l be fine if you know module 3 (transition elements) well, its not too hard, just read questions well

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Reply 108

tcameron

19

Original post by Saywhatyoumean

I thought we needed to say white compounds but colourless solutions?

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A solution is as a compound I think
Never heard this white precipitate thing

Reply 109

Also stuck on this question, i get everything else of the question apart from finding the conc of the undiluted h2o2 and dont understand how we use the moles of it to get its conc?

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Reply 110

on the previous page, it gives you the overall equation for this reaction which you need for molar ratios etc.

First thing you do is calculate moles of KMnO4 so use vol x conch to do so giving you 4.69x10-4

Then the molar ratio of 2MnO4 : 5H2O2 so you multiply the moles by 2.5 to get moles of hydrogen peroxide but this is only in 25cm3 but we need the concentration in the undiluted sample so this needs to be multiplied by 10 giving us 0.011725 moles.

Now we can calculate concentration using miles/volume so moles are 0.011725/(0.025) = 0.469 mol dm-3

But the units in the question are g dm-3 so multiply the conc by the Mr of the H2O2 so 0.469 x 34 = 15.9 g dm-3

@zirak46

Reply 111

Can anyone explain how to get the products of stage one? Please and thank you :smile:

Reply 112

ReeceM1

12

Original post by Saywhatyoumean

I thought we needed to say white compounds but colourless solutions?

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Original post by tcameron

A solution is as a compound I think
Never heard this white precipitate thing

My class was taught that a non-transition metal element would form a colourless solution, but as a precipitate it would be white.

E.g. If you mixed Zn^2+ + NaOH it produces Zn(OH)2 which is a white precipitate.
A physical property of transition elements is that they form coloured compounds. As zinc isn't a TM it forms white compounds/colourless in solution.

Reply 113

Took me a while to understand from markscheme (obvs didnt get in first go), hope it makes sense, reply again if it doesnt.

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Reply 114

I understand upto getting the conc of h2o2 but dont understand why we times by Mr?
Is there a formula that im missing?
n=cv, n=v/24, n=mass/mr
Dont understand why we do c=mass/mr

Shouldnt it mean by that logic 0.469 x 34 x (0.025)
Because cv = mass/mr

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Reply 115

Im at same stage as you, i literally started past papers today and i was so confused! Its just trial and error, do past papers and get them wrong, thats all good because this is just practise. Just learn from mark scheme and if it doesnt make sense, post them here, thats what im doing, posted 2 above so dont panic, we have time!

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Reply 116

Chill fam, take it steady, practise and go through your past papers every other day

Reply 117

From what i understand:

Kc is only affected by temp, thats it.
But when pressure or conc changes, kc is altered and so each of the top and bottom of kc expression either increase or decrease until Kc is reached again.

What page in the book were you takking about? Is this the Cambridge Ocr Chem book? If so, what page and i can try to help more?

Reply 118

Original post by zirak46

I understand upto getting the conc of h2o2 but dont understand why we times by Mr?
Is there a formula that im missing?
n=cv, n=v/24, n=mass/mr
Dont understand why we do c=mass/mr

Shouldnt it mean by that logic 0.469 x 34 x (0.025)
Because cv = mass/mr

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Because the units are in g mol-3 not in mol dm-3 so you have to multiply the concentration by mr to get it to g dm-3.

Reply 119