Chem qu (1 Viewer)

[Dreamerish*~]

how do go from ph=-loge[H+]
to
[H+]=...

what does [H+] equal i just cant remember...

pH = -log₁₀[H⁺] (Note: log base 10, not e!)

[H⁺] = 10^-pH

[H⁺] is the concentration of hydrogen ions in solution.

 

[richz]

im assuming most band 6 ppl get 100% in multi?

nah i dont think so they mite ask very challenging ones and also band 6 students might do silly mistakes

PS does anyone have a moles calculator?? i need to show someone sumthing

 

[Ghost1788]

lol me stupid... thanks people

 

[velox]

Someone remind me how to do these.

 

[richz]

i think its B, not sure because this mite have followed the old periodic table.

so we go like this 8/32 *24.79 (this value is the new one from the periodic table)

 

[velox]

yeh close enough, thats right.

 

[richz]

can sumone plz explain to the thread starter how to do this q. i dont know how i could explain it better

http://www.boredofstudies.org/community/showthread.php?p=1764436#post1764436

 

[velox]

<a href="http://img150.imageshack.us/my.php?image=chemqu8ob.jpg" target="_blank"><img src="http://img150.imageshack.us/img150/3618/chemqu8ob.th.jpg" border="0" alt="Free Image Hosting at www.ImageShack.us" /></a>
What about this one, its either a or b.

 

[richz]

cant be A. has to be B. i think u got a bit fooled there look at pb it forms a precipitate with hcl

 

[nit]

I'm not sure what conditions they give you molar volumes for - 24.79 looks like theyve given you molar volumes at 298.15K, 100kPa. In this case, 100kPa isn't strictly atmospheric pressure, but rather 101.3 is. The molar coefficient comes out to be 24.47 L/mol which gives 6.12L. It wouldn't have mattered too much given this was a multiple choice q.

Edit: the way you arrive at these coefficient values is beyond the scope of the course. But it comes from the empirically-derived equation:

PV=nRT, P=pressure, V=volume in litres, n=no. moles, R=ideal gas constant, T=Kelvin Temp. Put P in kilopascals, then R=8.314; if P is in atmospheres, then R=0.08206. Plugging in the right values gives you the values 24.79 and 24.47

 

[Dreamerish*~]

Someone remind me how to do these.

8.00 g of sulphur is burnt.

n = ^m/_M = ⁸/_32.1 = 0.249

Mole ratio = 1:1:1

Hence 0.249 moles of SO₂ is released.

Under standard conditions stated, 1 mole of gas occupies 24.79 L.

Volume = 0.249 x 24.79 = 6.18 L (2 d.p.) which is closest to answer b).

 

[velox]

oh yeh crap, shouldnt have rushed through the multi choice in 6.5 mins.

I guessed this one, do i just have to remember those flowcharts?

 

[richz]

my guess is C, not so sure abt this one, like 70% sure

 

[velox]

yeh its c, how did u get it?

edit: is it because it will form an insoluble compound ?

 

[richz]

insoluble = forms a precipitate, so only carbonate forms a precipitate with with cu, others dont so therefore u have identified its carbonate. i think this question is very vague. But i guess u have to pick to most correct answer and carbonate is prolly the most correct

 

[Dreamerish*~]

can sumone plz explain to the thread starter how to do this q. i dont know how i could explain it better

http://www.boredofstudies.org/community/showthread.php?p=1764436#post1764436

The reaction of sulphur during burning coal is:

S + O₂ → SO₂

10 million kg of coal was burnt. Therefore 10,000,000 x 0.001 = 10,000,000 g of sulphur was burnt.

n = ^m/_M = ^10,000,000/_32.1 = 311526.48 moles.

One mole of gas under the stated standard temperatures occupy 24.79 L.

Therefore volume = 311526.48 x 24.79 = 7722741.43 L

 

[richz]

ok thats alrite but u forgot to change it to g. anyway he/she doesnt understand how to get from moles to volumes. do u have a moles calculator to show her how to get to L.

 

[Dreamerish*~]

insoluble = forms a precipitate, so only carbonate forms a precipitate with with cu, others dont so therefore u have identified its carbonate. i think this question is very vague. But i guess u have to pick to most correct answer and carbonate is prolly the most correct

Not really.

It isn't:

a) Because nitrates do not form precipitates.

b) Because chlorides do not form precipitates except for silver and lead - which are stated.

d) Because sulphates do not form precipitates except for barium, lead, silver and calcium as stated.

That leaves you with c).

 

[richz]

but if u think about it, other substances can form a precipitate with carbonate, so it doesnt really have to be copper

 

[Dreamerish*~]

ok thats alrite but u forgot to change it to g. anyway he/she doesnt understand how to get from moles to volumes. do u have a moles calculator to show her how to get to L.

Oh, thanks for pointing that out, smart guy.

At 0ºC and 1 atm, one mole of any gas occupies 22.41 L.
At 25ºC and 1 atm, one mole of any gas occupies 24.79 L.

Is that it?