OCR Salters Chemistry F335 12th June 2013 Exam revision thread

yeah that is a bit confusing and it doesn't go into any detail but the way my teacher explained it was:

C is always halved if you double the volume it's in

C = n/V

as volume is the number on the bottom - as volume increases, the concentration decreases.

As they are in equal volumes - each concentration will be n / both their volumes added together (C = n/2V) which is the same essentially as halving the concentration.

The best way is to just pick any number for the volume and then find the moles and then double the volume and you'll find the concentrations halved. like picking randomly 1050 for the volume 0.1 = n / 1050 so n = 105. if the same conc is mixed in 1050 of something else it'll double the volume so 0.1 = n/2100 = 105 /2100 = 0.05

there is probably some mathematical and/or easier way to explain that better but thats how I understood it anyways and you can try with any number and the C is always halved if you double the volume

hope that helped !!

yeah that is a bit confusing and it doesn't go into any detail but the way my teacher explained it was:

C is always halved if you double the volume it's in

C = n/V

as volume is the number on the bottom - as volume increases, the concentration decreases.

As they are in equal volumes - each concentration will be n / both their volumes added together (C = n/2V) which is the same essentially as halving the concentration.

The best way is to just pick any number for the volume and then find the moles and then double the volume and you'll find the concentrations halved. like picking randomly 1050 for the volume 0.1 = n / 1050 so n = 105. if the same conc is mixed in 1050 of something else it'll double the volume so 0.1 = n/2100 = 105 /2100 = 0.05

there is probably some mathematical and/or easier way to explain that better but thats how I understood it anyways and you can try with any number and the C is always halved if you double the volume

hope that helped !!

also: looking at page 79 it seems like thats a mistake lmao and meant to say C3H8O

Sorry, but what's n and how do you know that the volume has been doubled? We're not given any information on volume

I thought so, but they've done it twice so I thought I must just be going mad!

Under alkali conditions your acidic group would be affected so the carboxyl group would deprotonate to form COO- which could form a salt yes. Basic groups are ones which have lone pairs of electrons so they are willing to accept a proton by forming a dative covalent bond.

n is number of moles you should be familiar with n = c x v

You are given that there are equal volumes of two solutions. So then when mixed together the volume of both together is double either one on it's own (sorry I threw in unnecessary numbers just to try and illustrate through the formula)

as each solution is diluting the other by a factor of 2, you therefore need to halve the concentration.

(if you do maths I've just figured out a way to show it mathematically lmao)

Oh right, I usually just use m
No, the formula makes sense, but I just don't understand how you know to half the volumes :/

well it makes sense using the formula and to remember if you times the volume by 2 you divide the concentrations by two
just like if you had 4 different solutions of equal volumes mixed, you'd divide each concentration by four and if you had 5, divide by 5 etc.

This is where i'm confused, where does it tell us that the volume was multiplied by 2?

sorry I'm not very good at explaining xD

if two volumes are equal and put together then it is the same as timesing one of the volumes by 2

the same way 10 + 10 = 10 x 2 :P

so it's the "two volumes that are equal" part which lets us know that the volume of each concentration is doubled when they're thrown together like if you mixed two seperate jugs of water of 4L each you'd have a final volume of 8L. So the volume of the mixture (the buffer solution) is therefore each volume added together, and if they are equal then it is the same as double the volume of one. each solution is basically diluting the other - does any of that help?

This bit has made me understand it. So if you have 10L of 6moldm orange juice and add 10L of water, you end up with 20L of 3moldm of orange juice, correct?

yeah man thats the idea - and if the water was instead 10L of 4moldm apple juice you'd have 3moldm of orange and 2moldm of apple in the 20L - I'm so sorry for confusing you with the rest of the stuff- I think you should look earlier in this thread cause someone explained it to me but I didn't understand in terms of diluting

can someone PLEASE help me with this question 5(f)(ii)

http://www.ocr.org.uk/Images/79341-question-paper-unit-f335-chemistry-by-design.pdf

thanks in advance!


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Admitted I had to reverse engineering from the markscheme to get this (i'm not sure if this is a proper explanation) but here we go.

We're told that the solution is mixed with 1/3 of the volume needed to neutralise; therefore the ratio of [salt] to [acid] is 1/3:2/3

Using Ka = [H+] * [salt]/[acid], substituting values into [salt]/[acid] leaves this to equal 1/2.

Then doing -log(2*Ka) = 2.73 which is your answer (I don't think this is entirely right as I think the answer is something to do with the extent of neutralisation between the mixtures too). Hope this helps

can someone PLEASE help me with this question 5(f)(ii)

http://www.ocr.org.uk/Images/79341-question-paper-unit-f335-chemistry-by-design.pdf

thanks in advance!


Posted from TSR Mobile

Do we just ignore the given concentration values for a question like this?
I can't figure out how to get from the concentration values given --> 1/2

Right I think I've eventually got it, I reckons its due to the fact that we're told 1/3 of the solution is diluted, and the ratio of [salt]/[acid] HAS to be 2 (from reverse engineering the answer).

So my final calculating came to be:

(0.0045*0.1)/((2/3)*0.05*0.027) which works out to be 1/2 exactly. I think the trick of this question is that the concentration of the acid (HA) has been partially diluted.

aaaaaaahhh you're the best man thanks very much - I was missing out the 2/3

can't believe it was only 1 mark!!