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# GCSE AQA Unit 3 Chemistry Titrations help Watch

1. Hey guys. I'm new here, so if I made a mistake in choosing the correct forum section or something, I apologise. My question: As I was doing some past papers in preparation for my AQA Unit 3 Chemistry exam in May, I found one where the answer I got disagrees with the examiner's answer. Here it is
"In a titration, 25.00cm^3 of a solution of aminoethanoic acid reacted with 18.40cm^3 of sodium hydroxide solution of concentration 0.15 moles per cubic decimetres. Calculate the concentration of aminoethanoic acid in moles per cubic decimetre"
The equation tells me that 1 mole of aminoethanoic acid reacts with 1 mole of NaOH. Here's what I did.
1.) Calculated the moles of NaOH so - 0.15 x 18.40/1000 = 0.00276 moles
2.) 1:1 ratio so 0.00276 moles of aminoethanoic acid
3. Formula- molarity = moles/(volume/1000) so 0.00276 / (25/1000) = 0.29 mol/dm^3
The answer the exam board gives is 0.11 mol/dm^3, saying that a mark would be given for calculating (0.15 x 18.40)/25. Could anyone help me as to where I've gone wrong? Thanks
P.S. Apologies in advance for the extremely long post!
2. Right, you did the moles correctly. 18.4/1000 x 0.15 = 0.00276.
To work out concentration, the equation is:

moles x 1000/ total or original volume (which is 25cm3) = 0.11 moldm-3
So you do 0.00276 (which is the moles of solution) x 1000/ 25 (which is volume of original solution) which should give you 0.11 moldm-3.

I think it's just a case of learning how to rearrange the equation, and you'll be golden.

Moles of a solution: vol/1000 x conc
Concentration of a solution: moles x 1000/vol
Volume of a solution: moles x 1000/conc

Bit fiddly, but there you go. You use the information you've got already, sub them into the equation to gather the missing piece of information.
3. Thank you, I understand it now . Just one last thing, is there a way of selecting a best answer or closing a thread?
4. (Original post by sbzk101)
Thank you, I understand it now . Just one last thing, is there a way of selecting a best answer or closing a thread?
Nope to selecting a best answer :P It's not like Yahoo!
You can either let the thread die, which it will as people will notice it's been resolved, or you can ask a Moderator to lock it but there'd really be no point to that.
5. Ok, thanks. I realise that the question was quite an obvious one, considering, as you said, this isn't Yahoo answers

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