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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: man0005 on February 03, 2011, 08:54:10 pm

Title: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 08:54:10 pm: Consider the combustion of biogas, in excess oxygen, in an engine designed to generate electricity. If the combustion is 100% efficient, CH4 is the only constituent of the biohas and the only products are CO2 and H2O.
a) What volume of gas products will be formed at 927 degrees and 110kPa if 25 kg of CH4 is burnt?
b) If the gas products are then cooled to STP, what volume will they now occupy?>

a) I think water is a gas product, so i got
v ( CO2) = 141,716
v (H2O) = 283,432

b) v ( CO2) = 35,009.3
v (H2O) = 76,430.5

Any help/corrections would be greatly appreciated :)
Title: Re: Gas Stoichiometry Question
Post by: m@tty on February 03, 2011, 09:19:56 pm: $pV=nRT$

At that temp, H2O is definitely a gas.

$CH_4(g)+2O_2(g) \to CO_2(g)+2H_2O(g)$

$n(CH_4)=\frac{25000}{16}=1562.5\text{ mol}$

And from the reaction CH4 : products = 1:3

So, $n(\text{gaseous products})=4687.5\text{ mol}$

$V=\frac{nRT}{p}=\frac{4687.5 \times 8.31 \times (927+273)}{110}= 424,943.182 = 4.25 \times 10^5 \text{ L}$ (3 sf)

Cooled to STP, T=273K, p=101.3 kPa. Work by ratio:

$V=424,943.182 \times \frac{110 \times 273}{101.3 \times 1200}= 104,977.326 = 1.05 \times 10^5 \text{ L}$ (3sf)
Title: Re: Gas Stoichiometry Question
Post by: simonhu81292 on February 03, 2011, 09:22:25 pm: i got something different ...
but it's very similar to yours..~~
my answers are:
a)
CO2= 141,647L
H2O=283,295L
b)
CO2= 34,992L
H2O=69,984L
;)
Title: Re: Gas Stoichiometry Question
Post by: simonhu81292 on February 03, 2011, 09:24:38 pm: 25000/16= 6250..haha
1562.5..that's what i've got
Title: Re: Gas Stoichiometry Question
Post by: m@tty on February 03, 2011, 09:26:21 pm: Oh lol. I shall rectify that :P
Title: Re: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 09:37:18 pm: oh matty
doesnt the pressure change to STP conditions as well when its cooled or is it just the temp?
Title: Re: Gas Stoichiometry Question
Post by: m@tty on February 03, 2011, 09:41:59 pm: Yes, yes it does. How foolish I am. =\
Title: Re: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 09:46:49 pm: oh okay
how come our answers for water in b) are so different simon O_O
Title: Re: Gas Stoichiometry Question
Post by: m@tty on February 03, 2011, 09:49:07 pm: Mine and simons numbers now agree after I corrected the pressure in b)..
Title: Re: Gas Stoichiometry Question
Post by: simonhu81292 on February 03, 2011, 09:54:08 pm: yeah ..i know ...i was surprised as well..
but i double checked ...
let's wait for someone else for confirmation

thanks matty~~!
Title: Re: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 09:55:32 pm: ah i used R = 8.31 instead of R = 8.314 for a)
and got 141648 L for CO2
think that must be it
does it matter much?
Title: Re: Gas Stoichiometry Question
Post by: m@tty on February 03, 2011, 10:05:30 pm: I only used 8.31?

Not that it matters now, but always use the values given in the data book for the exam (obviously). We also used it for SACs at my school.. Friendly advice from a moderator (atone for my terrible attempt at answering your question :P haha)
Title: Re: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 10:09:24 pm: oh okay
yeah I used 8.314 thats why it was different I think
i think i'll leave it since i've used 8.314 for everything else. hopefully teacher isnt too mean considering this is the first piece of hw :P

if you guys have some time, i have one more quick question
A certain pharmaceutical company produces a medication that contains 40.0 mg of magnesium hydroxide, Mg(OH)2, per ml. This is the only consitituent of the preparation with acid-base properties.

Calculate the concentration of hydroxide ions in the mixture
i got 1.37 M
Title: Re: Gas Stoichiometry Question
Post by: m@tty on February 03, 2011, 10:19:35 pm: In one litre there will be 40g of magnesium hydroxide.

$n(Mg(OH)_2)=\frac{40}{24.3+34}=0.686\text{ mol}$ (this is the same as the concentration in M as it is the mol in a litre)

So $[OH^-]=2\times n(Mg(OH)_2)= 1.37 M$ (3sf) :) So you're correct.

EDIT: I like my post-count :P - it was 4040 :coolsmiley:
Title: Re: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 10:24:09 pm: om goshh, thanks heaps :D
do you have any time atm to help me with a few physics questions?
Title: Re: Gas Stoichiometry Question
Post by: m@tty on February 03, 2011, 10:29:50 pm: Post it in the physics board and someone will help you out :)
Title: Re: Gas Stoichiometry Question
Post by: simonhu81292 on February 03, 2011, 10:33:24 pm: lol...some kind of test from this guy ..chem then physics...
let's jump to the physics thread~~!!
Title: Re: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 10:34:27 pm: LOL lets go ;)
Title: Re: Gas Stoichiometry Question
Post by: man0005 on February 03, 2011, 10:42:34 pm: http://vce.atarnotes.com/forum/index.php/topic,37733.msg399604/topicseen.html#msg399604
:DDDD
woo