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# AS Chemistry- helping each other out! watch

1. (Original post by Joshthemathmo)
I've heard of the compound but i've not encountered that mechanism! I'm on AQA by the way
Ah I see!! You know now
2. Hey guys I'm stuck on this one question.

The fuel tank of one type of hydrogen-powered car holds 70kg of magnesiumhydride. Calculate the volume of hydrogen gas, measured at room temperature and pressure,which would be produced if this amount of magnesium hydride reacted with water.

MgH2(s) + 2H2O(I) -----> Mg(OH)2(s) + 2H2(g)

The marking scheme says this :

Moles MgH2 = 70000 = 2659.6 (2660) (1)

26.32Moles H2 = 5319.2 (5320) (1) I've gotten to here, but I just cant figure out what to do to get the next answer.

Volume H2 = 1.28 × 105dm3(1)

Thanks for any help!
3. can someone explain geological and archaeological dating please? like in a lot of depth so i have full detail in case a question comes up. Thanks
4. also what are the benefits and risks of using radioactive tracers?
5. (Original post by Amethyst-Wolf)
Hey guys I'm stuck on this one question.

The fuel tank of one type of hydrogen-powered car holds 70kg of magnesiumhydride. Calculate the volume of hydrogen gas, measured at room temperature and pressure,which would be produced if this amount of magnesium hydride reacted with water.

MgH2(s) + 2H2O(I) -----> Mg(OH)2(s) + 2H2(g)

The marking scheme says this :

Moles MgH2 = 70000 = 2659.6 (2660) (1)

26.32Moles H2 = 5319.2 (5320) (1) I've gotten to here, but I just cant figure out what to do to get the next answer.

Volume H2 = 1.28 × 105dm3(1)

Thanks for any help!
The answer is expressed in index form. Your value should be somewhere around 128,000 dm3 in standard form. (1.28 * 10^5)

After you have correctly gotten the moles of H2(g) which is 5319.2 using mole ratio from equation and mol of MgH2 (from your periodic table). Multiply that value by the molar volume at room temperature and pressure (RTP) which is always 24 dm3. You should get: 127,660.8 dm3 which is close enough. You can round up to get 128,000dm3. You can multiply that by 1000 to convert to cm3 if you want.

So it's (mol of product) * 24 dm3 (Vm) = V
6. Thanks _NMcC_ ! Thats super helpful
7. (Original post by RagaZ)
Saw you guys were a bit confused... One of the above posters are right, Br2(aq) is bromine water, Br-OH? Nope

I've drawn the mechanism below and am pretty certain it's right, this stuff comes up in A2 too

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That's wrong. Bromine adds anti to a double bond

EDIT: it should be like this
Attached Images

8. Anyone have got paper 2015

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9. (Original post by annie79)
when bromine water (Br-OH) is added to an alkene, is it the Br or OH that is the nucleophile in the mechanism for the reaction?
(Original post by C0balt)
Br2+H2O->BrOH+HBr

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The Alkene is the nucleophile. The Alkene reacts with the Br-Br. How could BrOH even react? It would dissociate as BrO- and H+. And that equilibrium lies well to the left anyway.
10. Just a note: you'll lose marks if the curve arrow is not touching the bond
11. (Original post by langlitz)
The Alkene is the nucleophile. The Alkene reacts with the Br-Br. How could BrOH even react? It would dissociate as BrO- and H+. And that equilibrium lies well to the left anyway.
Well I don't have that much knowledge I'm afraid
I just remember my teacher saying bromoalcohol forms with bromine water because HOBr inside bromine water whatnot

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12. (Original post by C0balt)
Well I don't have that much knowledge I'm afraid
I just remember my teacher saying bromoalcohol forms with bromine water because HOBr inside bromine water whatnot

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HoBr doesn't form. In AQ the bromine adds to one carbon of the double bond forming a carbocation as normal. The bromine is AQ so compared to the other nucleophile present (water) there is very little of the bromine - ions. Therefore the major product involves the water adding to the carbocation and then the extra hydrogen reacts with a lone pair from another water molecule forming an H3O (hydronium) ion

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13. (Original post by langlitz)
That's wrong. Bromine adds anti to a double bond

EDIT: it should be like this
This is A-level, it's a simplified version. What RagaZ drew is perfectly acceptable (at least for AQA)
14. (Original post by samb1234)
HoBr doesn't form. In AQ the bromine adds to one carbon of the double bond forming a carbocation as normal. The bromine is AQ so compared to the other nucleophile present (water) there is very little of the bromine - ions. Therefore the major product involves the water adding to the carbocation and then the extra hydrogen reacts with a lone pair from another water molecule forming an H3O (hydronium) ion

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thats makes more sense
teacher wtf
15. resitting unit two even though I got 83/100 (which was just an A), to get a higher A.
Forgotten most of the content, so I'm starting my revision for this now
16. (Original post by C0balt)
thats makes more sense
teacher wtf
Yeah I found what you were talking about but there is definitely no BrOH involved
17. (Original post by Dylann)
This is A-level, it's a simplified version. What RagaZ drew is perfectly acceptable (at least for AQA)
Well it's wrong never the less. Pretty sure we did it in advanced higher
18. (Original post by langlitz)
Yeah I found what you were talking about but there is definitely no BrOH involved
Brb I'll go smack my teacher's head

Jk lol
Thanks anyway haha

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19. I posted a thread earlier but does anyone want to study f322 with me? Over chat ( kik/Facebook etc ) haha

Id find it really helpful if we set an amount of work to do and then test one another, discuss past paper results etc

I need motivation and to have someone to discuss things with would really help

Would anyone be interested?

ps idm who but someone who is doing last minute revision etc, not someone who knows the whole spec already

also for f324, if anyone is interested

Let me knowww
20. (Original post by langlitz)
Well it's wrong never the less. Pretty sure we did it in advanced higher
No it's not "wrong" because it would get you the marks in an AS exam. Hey, look at the title of this thread, it says AS!! Not Advanced Highers!

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