Back Titrations AQA

Hi, i was given a booklet through school for A Level AQA chemistry and it contained back titration calculations, however I cannot find anything to do with these on the spec. My teacher has a history of giving out sheets and work that is made for different courses 😂
Does anyone doing AQA A Level Chem know if back titration calcualtions are apart of our course and exams?

cheers
M

Reply 1

BDavies1

8

Original post by Macdeez44

Hi, i was given a booklet through school for A Level AQA chemistry and it contained back titration calculations, however I cannot find anything to do with these on the spec. My teacher has a history of giving out sheets and work that is made for different courses 😂
Does anyone doing AQA A Level Chem know if back titration calcualtions are apart of our course and exams?

cheers
M

A back titration is where you react something (e.g. CaCO3) with a known amount of (in excess of another substance e.g. HCl). Then you do a titration to see how much is left of the original (in this case HCl). This enables you to work out how much (in this case CaCO3) was added to the acid.

There is no reason why these sort of calculations should not appear on AQA papers. They asked one in paper 1 June 18

See below

Reply 2

Macdeez44

OP

9

Original post by BDavies1

A back titration is where you react something (e.g. CaCO3) with a known amount of (in excess of another substance e.g. HCl). Then you do a titration to see how much is left of the original (in this case HCl). This enables you to work out how much (in this case CaCO3) was added to the acid.

There is no reason why these sort of calculations should not appear on AQA papers. They asked one in paper 1 June 18

See below

Ah thank you that is helpful,
Ill give that question a go, 7 marks is alot!

Reply 3

BDavies1

8

Original post by Macdeez44

Ah thank you that is helpful,
Ill give that question a go, 7 marks is alot!

Good luck with it- it one of the most challenging ones they have set.

Reply 4

Macdeez44

OP

9

Found its mark scheme, not to sure why they x10 in M2 stage? very confused by that, hoping its cause of my Yr12 Ignornace!
Back Titration.png

Reply 5

BDavies1

8

Original post by Macdeez44

Found its mark scheme, not to sure why they x10 in M2 stage? very confused by that, hoping its cause of my Yr12 Ignornace!
Back Titration.png

Hi,
You have to x by 10 because you only titrate 25 cm3 of the HCl solution, while the total volume is 250 cm3.

This Q is from an A-level paper you take at the end of year 13 , and so it's hard for year 12's!

I would suggest the following approach:
1. calculate the moles of NaOH in the titre (conc x vol)
2. The equation for the titration is NaOH + HCl --> NaCl + H2O so 1 mole of NaOH reacts with 1 mole HCl
3. This will tell you moles of HCl in the 25 cm3 in the titration
4. This 25 cm3 of HCl came from a volumetric flask of 250 cm3, so to find the moles of HCl in the volumetric, x by 10
5. The moles of HCl in the volumetric is the moles HCl left over after the Na2CO3 has reacted with it, call this moles HCl f (f for final)
6. Now work out the start moles of HCl (i'e moles of HCl before any Na2CO3 has been added). Call this moles HCl s (s for start). This number of moles can be calculate by conc x volume (= 0.25 x 200/1000)
7. Work out the moles of HCl u (u for used up) by the Na2CO3, by HClu = HCls- HClf
8. Equation for reaction of Na2CO3 with HCl is Na2CO3 + 2HCL --> 2NaCl + CO2 + H2O, so moles Na2CO3 = moles HClu/2
9. Know you know i. moles Na2CO3.xH2O and ii mass Na2CO3.xH2O so using Mr = mass/moles, you can work out the Mr of Na2CO3.xH2O. From this you can work out x i.e. how many moles of H2O in the formula.

P.S. for AQA please check out my YouTube videos " Davies A-level chemistry" . All of AQA spec covered.... some AQA biology A-level too.

Back Titrations AQA

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