AS Chemistry- helping each other out!

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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

Reply 2521

Amethyst-Wolf

Thanks _NMcC_ ! Thats super helpful :smile:

Reply 2522

langlitz

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

Posted from TSR Mobile

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

EDIT: it should be like this

(edited 8 years ago)

image.jpg434.2KB

Reply 2523

Hadiqa malik

Anyone have got paper 2015

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Reply 2524

langlitz

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

Posted from TSR Mobile

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.

Reply 2525

Coveted Ul

Just a note: you'll lose marks if the curve arrow is not touching the bond

Reply 2526

C0balt

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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Reply 2527

samb1234

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

Posted from TSR Mobile

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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Reply 2528

Dylann

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)

Reply 2529

C0balt

Original post by samb1234

thats makes more sense
teacher wtf

Reply 2530

NutE

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 :biggrin:

Reply 2531

langlitz

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

Reply 2532

langlitz

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

Reply 2533

C0balt

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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Reply 2534

hibaj

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 :tongue:

also for f324, if anyone is interested

Let me knowww

(edited 8 years ago)

Reply 2535

Dylann

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!

Reply 2536

samb1234

Original post by Dylann

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!

It is technically correct anyway as you will get some dibromoalkane formed but the major product would be a bromoalcohol

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Reply 2537

User1686961

Original post by NutE

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 :biggrin: