Aqa chem4 15th june 2011 (resit) thread

You know for those big 3 marker questions .....where you had name reagents....etc.
One of them showed an aldehyde.....Is it ok if I put tollens' reagent and silver mirror is formed?

Omggggggggg i thought the exam went ok until i looked on this thread !

You know for those big 3 marker questions .....where you had name reagents....etc.
One of them showed an aldehyde.....Is it ok if I put tollens' reagent and silver mirror is formed?

What did you guys put for the question about 'what would need to be done for the public to find this more reliable' or something like that, was to do with pthlates question I think..

I hated this question! Really didn't know what to write

Calculate the pH of the solution formed when 25.0cm3 of 0.150 moldm–3 aqueous sulfuric acid are added to 30.0cm3 of 0.200 moldm–3 aqueous potassium hydroxide at 25 °C. Assume that the sulfuric acid is fully dissociated.

It wants you to give the pH of the solution after the two substances have reacted.

pH = -log[H+]

So we need to find the concentration of protons when the two solutions have reacted.

Find the initial number of moles of H+ (vol. x conc. x 2 as sulfuric acid is diprotic (two protons per molecule))
25/1000 x 0.15 x 2
= 7.5 x 10 to the power of minus 3 moles of H+.

find the initial moles of OH- (vol. x conc.)
30/1000 x 0.2
= 6 x 10 to the power of minus 3 moles of OH-.

Find out which is in excess (in this case the protons) and subtract the number of moles of the other substance from it. In essence the protons and hydroxide ions react to form water, leaving behind a certain number of moles of the substance in excess (in this case H+) which have nothing to react with.

So (7.5 x 10 to the power of minus 3) - (6 x 10 to the power of minus 3)
= 1.5 x 10 to the power of minus 3 moles of H+ left.

We then convert this to concentration.
Conc = moles / volume

The volume is the total volume after the reaction (i.e. the two solutions have been added together so the volumes are added together).
Volume in dm3 = (25 + 30) / 1000
= 0.055

Thus conc. = (1.5 x 10 to the power of minus 3) / 0.055
= 0.0273moldm-3 to three sig. fig.

-log(0.0273)
= 1.56

Hope this is helpful to you!

cos it said the organisers of the report represented the manufacturers of the phthalate stuff, i said that they should get an independant body to test the product as the results above might be represnted in a way favouring the manufacture of phthalates.... anyone else put somehting like tht?

Which was it that was a Aldehyde I don't remember seeing it on there?

I remember a ketone though, pls remind me

I hope so haha! thats what i put... I reckon its one mark for tollens, then one mark for the aldehyde forms a silver mirror and one mark for saying the other compound will have no reaction

SAME! I remember a ketone but NO aldehyde!!!

Maybe people on here read it wrong it looked like a classic Tollens distinguish question and it seems people rushed! there was no aldehyde it was an Acyl Chloride.

73 was an A in january

this happened to me aswell - it makes you feel like CRAP!!!

i think people thought part i was the aldehyde and ketone, when infact it was carobxylic acid and ester??? is this correct

SAME! I remember a ketone but NO aldehyde!!!

First two were Ester and carboxylic acid
Second pair were acyl chloride and something else
Third pair was a primary alcohol and a tertiary alcohol