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GCSE OCR 21st Century Chemistry C7

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Reply 60
Avatar for kxylah
kxylah
OP
9
Original post by abbeyjayne
could you explain the reflux stuff to me please? i still struggle to get it aahh.

in general, i'm feeling pretty nervous about this exam but even if it goes ****e, i have my leavers assembly tomorrow so that'll be a positive!


Reflux is basically the first step of making an ester, first you have a flask and you add in your alcohol, carboxylic acid and catalyst (usually concentrated sulphuric acid) and to start the reaction you need to heat the flask but if you just do it then the alcohol will evaporate before it can react so on top of the flask you add in a condenser so that if it does evaporate it will cool back into a liquid and react, thats basically called refluxing, hope that helped! :smile:
Original post by swagmister
Could someone explain titrations finding the concentration. Are we supposed to know how to find the concentration or mass or both?


Remember these two formulas

Concentration = Mass/Volume

Concentration of acid =(Volume of alkali x Concentration of alkali)/ volume of acid

You just need to rearrange.

When I do a titration calculation I always follow these steps.

1 - Wrote out word equation
2 - Wrote out balance symbol equation
3 - Work out ratio of acid to alkali
4 - Work out relative formula masses
5 - Create a table for acid and alkali
6 - find out unknowns

One thing that thrown me was the relative formula mass here is an example question.

A titration is carried out and 35cm^3 sulfuric acid of concentration 60g/dm^3 neutralises 25cm^3 of sodium hydroxide. What is the concentration of sodium hydroxide.

1 - Work out rfm

Sulfuric acid has a formula mass of 98
Sodium hydroxide formula mass 40

The ratio of acid to alkali is 1:2
So we x sodium hydroxide mass by 2

2 - work out mass of sulfuric acid.

60g/dm^3 x (35/1000) * (convert cm^3 into dm^3)

= 2.1g

Work out mass of sodium hydroxide

(2.1g/98) x (40x2)

= 1.7g

Work out concentration

1.7/(25/1000)

= 68g/dm^3
Reply 62
Avatar for kxylah
kxylah
OP
9
one of the 6 marker predictions is "describe the procedure of quantitative analysis using titration"
idk what the question means?
Original post by Jitesh


Thanks for this, anywhere I can find the MS's?
Have been cramming all day after my History exam this morning, its all so hard! :frown:
Original post by kxylah
one of the 6 marker predictions is "describe the procedure of quantitative analysis using titration"
idk what the question means?


Quantitate analysis tells how much of a substance is present is a solution. So, I think it's when you're carrying out a titration you're seeing how much of an acid or alkali is used to neutralise the substance. I would just go through the steps of a titration and link to quantitative analysis saying how you have a definite measure of how much acid has been used, as thus can work out the concentration.
Original post by kxylah
one of the 6 marker predictions is "describe the procedure of quantitative analysis using titration"
idk what the question means?


Basically it means how do you carry out a titration and get the most accurate results. Step by step.

Choose your solutions
Measure them out
Dissolve then if necessary
Titrate the burette through to acid
Use indicator
Shake acid solution
Record results
Get a value
Work out mean
Get an average result.
Reply 67
Avatar for kxylah
kxylah
OP
9
Original post by The_Philosopher
Quantitate analysis tells how much of a substance is present is a solution. So, I think it's when you're carrying out a titration you're seeing how much of an acid or alkali is used to neutralise the substance. I would just go through the steps of a titration and link to quantitative analysis saying how you have a definite measure of how much acid has been used, as thus can work out the concentration.


Original post by jake2358
Basically it means how do you carry out a titration and get the most accurate results. Step by step.

Choose your solutions
Measure them out
Dissolve then if necessary
Titrate the burette through to acid
Use indicator
Shake acid solution
Record results
Get a value
Work out mean
Get an average result.


ohh yes i get it now, thanks!! :smile:
Original post by kxylah
Reflux is basically the first step of making an ester, first you have a flask and you add in your alcohol, carboxylic acid and catalyst (usually concentrated sulphuric acid) and to start the reaction you need to heat the flask but if you just do it then the alcohol will evaporate before it can react so on top of the flask you add in a condenser so that if it does evaporate it will cool back into a liquid and react, thats basically called refluxing, hope that helped! :smile:


Yes it did thanks! I read the revision guide too and that made sense. Quick Q also to you or anyone!? I'm doing the revision summary in CGP Chemistry guide and I'm on Q13 which is the balanced symbol equation for combustion of ethane in plenty of oxygen.

I put: C2H6 + 3.5O2 > 2CO2 + 3H2O
Yet the guide says the answer is: 2c2H6 + 7o2 > 6h2o + 4co2

My chem teacher said you could do halves so is my answer also right d'ya think?
Original post by kxylah
one of the 6 marker predictions is "describe the procedure of quantitative analysis using titration"
idk what the question means?



Hi . Please can you send your prediction questions?
Original post by Zaydo
Here is the 2014 paper and the mark scheme


Very difficult paper. The 6-mark questions were thankfully straightforward, but the rest of them were quite difficult. Especially the RFM calculations -- I thought they the easiest part of the syllabus, but the fact that I got 80% of those marks wrong shows that I need to go over them.

Still managed to get 45/60, which is a high A/low A*. I hope tomorrow's exam is easier, though. I need to create a cushion for an A* because I got a B in my coursework. :frown:
Reply 71
Avatar for Zaydo
Zaydo
8
Original post by abbeyjayne
Yes it did thanks! I read the revision guide too and that made sense. Quick Q also to you or anyone!? I'm doing the revision summary in CGP Chemistry guide and I'm on Q13 which is the balanced symbol equation for combustion of ethane in plenty of oxygen.

I put: C2H6 + 3.5O2 > 2CO2 + 3H2O
Yet the guide says the answer is: 2c2H6 + 7o2 > 6h2o + 4co2

My chem teacher said you could do halves so is my answer also right d'ya think?


Your answer is fine.
Reply 72
Avatar for Zaydo
Zaydo
8
Original post by ETbuymilkandeggs
Very difficult paper. The 6-mark questions were thankfully straightforward, but the rest of them were quite difficult. Especially the RFM calculations -- I thought they the easiest part of the syllabus, but the fact that I got 80% of those marks wrong shows that I need to go over them.

Still managed to get 45/60, which is a high A/low A*. I hope tomorrow's exam is easier, though. I need to create a cushion for an A* because I got a B in my coursework. :frown:


Yeah, couple fiddly questions in that one. I'm wishing that I can at least get an A in this one. Biology was a breeze, and I'm thinking physics shouldn't be that bad.
does anyone have the link for the 2014 june biology b b1 b2 b3 paper?
What are the advantages and disadvantages of the three methods of making ethanol?
Reply 75
Avatar for derpz
derpz
5
Could someone explain how you would work out the calculations which ask "what concentration of x is in y"?
Original post by asdfghjkldude
What are the advantages and disadvantages of the three methods of making ethanol?


Fermentation

Advantages -- raw materials grow quickly, relatively low temperature required (30 celcius)

Disadvantages -- waste CO2 produced (low atom economy), too slow for large-scale ethanol production

Biomass

Advantages -- made using part of plants which would normally be thrown away, crops don't have to be specifically produced for ethanol production

Disadvantage -- cannot be fermented regularly due to cellulose content, too slow for large-scale ethanol production

Ethene reaction with steam

Advantages -- production of high-quality ethanol continuously and rapidly

Disadvantage -- requires a high temperature and pressure to be used, which costs $$$
(edited 8 years ago)
Original post by Vanilla Poison
Thanks for this, anywhere I can find the MS's?


Do you know whether we need to know any chemical equations for the topic related to reversible reactions, equilibrium etc?

Thanks
Do we need to actually memorise the titration and concentration formulae or will they be in the paper?
Reply 79
Avatar for derpz
derpz
5
Original post by parker13
Do we need to actually memorise the titration and concentration formulae or will they be in the paper?


You need to memorise them.

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