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# A-level Chemistry Revision Squad! watch

1. Hey can anyone explain 23c to me?
Its from the june 2014 ial Chem 5 paper
Thanks x
2. (Original post by the-anonymous-me)

Hey can anyone explain 23c to me?
Its from the june 2014 ial Chem 5 paper
Thanks x
CxHy (10) + (x + y/4)O2 (v) -> (y/2)H20 (0) + xC02 (40)

The bold are the volumes.

Use ratios:

C02:CxHy = x = 40/10 = 4
To find the volume of Oxygen used, use this:

Change in volume = volume of gas produced - volume of gas used

Then again use ratios to find the value of
(x + y/4) and then solve for y.
3. (Original post by demotivated)
CxHy (10) + (x + y/4)O2 (v) -> (y/2)H20 (0) + xC02 (40)

The bold are the volumes.

Use ratios:

C02:CxHy = x = 40/10 = 4
To find the volume of Oxygen used, use this:

Change in volume = volume of gas produced - volume of gas used

Then again use ratios to find the value of (x + y/4) and then solve for y.
Thanks xx - I think I get it now
4. (Original post by the-anonymous-me)
Thanks xx - I think I get it now
No problem, though i'm having trouble with a similar question 18 on June 2014 R

"15 cm^3 of a gaseous hydrocarbon requires 90cm^3 of oxygen for complete combustion, both volumes being measured at 15C and 1 atm. The formula of the hydrocarbon is:"

a) C4H6
b) C4H8 (this is the answer)
c) C4H10
d) impossible to determine

anyone have any idea?
5. Does anyone know the full equation for fe(h2o)4(oh)2 with oxygen to form fe(h20)3(oh)3?
6. (Original post by demotivated)
No problem, though i'm having trouble with a similar question 18 on June 2014 R

"15 cm^3 of a gaseous hydrocarbon requires 90cm^3 of oxygen for complete combustion, both volumes being measured at 15C and 1 atm. The formula of the hydrocarbon is:"

a) C4H6
b) C4H8 (this is the answer)
c) C4H10
d) impossible to determine

anyone have any idea?
There may be a quicker way but this is how I did it:

The hydrocarbon: oxygen ratio is 15:90 which can be simplified to 1:6

CxHy + 6 O2 ----> x CO2 + 0.5y H2O

x + 0.25y = 6 This is by looking at the coefficients of O2 on the right

The only thing that satisfied this from the options given was B as

4 + 0.25(8) = 4 + 2 = 6
7. (Original post by the-anonymous-me)
There may be a quicker way but this is how I did it:

The hydrocarbon: oxygen ratio is 15:90 which can be simplified to 1:6

CxHy + 6 O2 ----> x CO2 + 0.5y H2O

x + 0.25y = 6 This is by looking at the coefficients of O2 on the right

The only thing that satisfied this from the options given was B as

4 + 0.25(8) = 4 + 2 = 6
Okay, thank you!
8. How do you do this question?

Posted from TSR Mobile
9. (Original post by Glavien)
How do you do this question?

Posted from TSR Mobile
Cr(OH)3 + 3OH- ---> Cr(OH)6
Cr(OH)3 + 3H+ ----> Cr(H2O)6

Posted from TSR Mobile
10. I know this is AS but really stuck any help/jints?
Direct titration

A student has read on a packet of aspirin that each tablet contains 250 mg of aspirin, and has decided to find out if this is true.

The systematic name of aspirin is 2-ethanoylhydroxybenzoic acid. Aspirin is a carboxylic acid and neutralises a cold dilute solution of sodium hydroxide to form a salt. This reaction is used to form the basis of a titration.

CH3COOC6H4COOH + NaOH → CH3COOC6H4COONa + H2O

aspirin

1. Calculate the relative molecular mass of aspirin (2-ethanoylhydroxybenzoic acid). CALCUALTED 180 (1 mark)

The student decides to use this reaction in a titration to determine the mass of aspirin in each tablet.

She crushes four tablets, transfers them to a 100 cm3 volumetric flask, dissolving them in a 50:50 mixture of ethanol and distilled water (it is difficult to dissolve aspirin in pure water). Using a pipette she transfers 25 cm3 of this solution into a conical flask and titrates it against a 0.1 mol dm−3 standard solution of sodium hydroxide, using phenolphthalein as an indicator. The average titre is 14.70 cm3.

1. Calculate the mass of aspirin in each tablet. (4 marks)

Not sure where to start
11. (Original post by iiiiiiiiiiiiiiii)
I know this is AS but really stuck any help/jints?
Direct titration

A student has read on a packet of aspirin that each tablet contains 250 mg of aspirin, and has decided to find out if this is true.

The systematic name of aspirin is 2-ethanoylhydroxybenzoic acid. Aspirin is a carboxylic acid and neutralises a cold dilute solution of sodium hydroxide to form a salt. This reaction is used to form the basis of a titration.

CH3COOC6H4COOH + NaOH → CH3COOC6H4COONa + H2O

aspirin

1. Calculate the relative molecular mass of aspirin (2-ethanoylhydroxybenzoic acid). CALCUALTED 180 (1 mark)

The student decides to use this reaction in a titration to determine the mass of aspirin in each tablet.

She crushes four tablets, transfers them to a 100 cm3 volumetric flask, dissolving them in a 50:50 mixture of ethanol and distilled water (it is difficult to dissolve aspirin in pure water). Using a pipette she transfers 25 cm3 of this solution into a conical flask and titrates it against a 0.1 mol dm−3 standard solution of sodium hydroxide, using phenolphthalein as an indicator. The average titre is 14.70 cm3.

1. Calculate the mass of aspirin in each tablet. (4 marks)

Not sure where to start
1. Asked for mass which is moles x Mr
2. Moles = volume x concentration
3. Titration is a neutralisation reaction so the moles of NaOH will be equal to the moles of aspirin
4. Calculate the mass
5. Divide by 4 (as 4 tablets were crushed)

Give it a go
12. can anybody check over this for me please? Thanks
14. (Original post by kiiten)
Why?
15. (Original post by Farmerjj)
Why?
Because exams are drawing closer but no one seems to post here. I mean if i get stuck now i just make a new thread. We're supposed to help each other out. The maths thread is always so active compared to this one.....
16. (Original post by kiiten)
Because exams are drawing closer but no one seems to post here. I mean if i get stuck now i just make a new thread. We're supposed to help each other out. The maths thread is always so active compared to this one.....
This subforum isn't very active so making a new thread is good enough. Just follow the subforum and all chemistry threads will pop up on your My TSR page.
17. Can someone help me work out this questions cause i have no clue, thanks x

A 25.0cm3 sample of 0.0850 mol dm-3 hydrochloric acid was placed in a beaker. distilled water was added until the pH of the solution was 1.25. Calculate the volume of the solution formed. (3)
18. (Original post by ashpittawayx)
Can someone help me work out this questions cause i have no clue, thanks x

A 25.0cm3 sample of 0.0850 mol dm-3 hydrochloric acid was placed in a beaker. distilled water was added until the pH of the solution was 1.25. Calculate the volume of the solution formed. (3)
What's pH?
19. Hi there is anyone able to help with this kp calculation 1.0 moles of nitrogen and 1.0 moles of oxygen are mixed an left to reach equilibrium if the value of kc is 0.0036 what mass of NO would be formed ?
N2-- 2N0
20. Hi everyone,
Does anyone know how to work out the number of molecular ion peaks in question 4b) of the following paper:

http://pmt.physicsandmathstutor.com/...%20A-level.pdf

Thanks!

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