2004 CSSA Trial Paper ... Mistake? (1 Viewer)

[Alexluby]

Has anyone done the 2004 CSSA HSC Trial papers??

I was just wondering for question 23...

"Coal, containing 0.1% sulfur, is burned in a power station. Calculate the volume of sulfur dioxide released at 25 degrees Celsius and 100kPa when 10.0 million kg of coal is burned."

Supposedly, to work out the mass of sulphur... shouldn't it shud be 0.1% x 10^7??

Coz the answer booklet showed that the mass of sulphur is worked about as 0.01% x 1 x 10^10 = 1 x 10^6

Can someone plz tell me if i have done something wrong?

 

[richz]

yes i agree with this too, i think there is a mistake in the answers

 

[Alexluby]

Cool... thx

 

[Dreamerish*~]

Maybe they messed up the zeroes. (They also spelt "volume" as "colume", or is it just you? )

 

[Alexluby]

lol yea... it's just me... hehe, well spotted.

Anyways, would anyone mind posted on the correct answers as well?

 

[richz]

keep on going, ((1 * 10^7 )*24.79)/32 = 7.75 * 10^6 L

 

[Alexluby]

keep on going, ((1 * 10^7 )*24.79)/32 = 7.75 * 10^6 L

I dun realli get what u did but is this right??

Mass of S:
0.1% x 10^7
= 10 000kg
= 10^7 grams

M(S) = 32.06
n= m/M
.'. n(S) = 10^7 / 32.06
= 311915.16

... hmm i got a feeling im wrong..

 

[richz]

yes keep on going that only gives u moles, u want litres so u times by 24.79

 

[Alexluby]

- "where did u get ur 24.79?"

anyway keep on going.....
what i would do is:
n=CV

but... we are trying to find V... and C is unknown... so.. am i supposed to do something else first?

 

[richz]

on the data sheet, hmmm u shud have a look at a moles calculator do u have one??

 

[Alexluby]

no.. are u saying that n = 24.97?

i thought u said
n= m/M
.'. n(S) = 10^7 / 32.06
= 311915.16
is right tho :S

 

[richz]

yeah that gives u moles. u need to find litres.

Does anyone have a moles calculator, the one with the circle and moles at the centre

it will be clearer when u see that

and i got the 24.79 from the data seet

 

[Alexluby]

can't we just use V = n/C ?

 

[richz]

i dunno, i dont remeber eqns for chem. And where would we get the concentraton from ???

 

[Alexluby]

Yea... exactly, that's why i dunno how to do it

But in da exam are we allowed to bring in a mole calculator or wuteva u said??

 

[richz]

nope. but its just about dealing with the units

 

[brightsea]

xrtzx, is right :uhhuh:

they've screwed up the answers. i've got a handwritten note on my answers that says, "each power of 10 should be 1 greater." but just to recap:

S(s) + O2(g) --> SO2(g)

1. Find the mass of S in the coal:

Mass of S = 0.1/100 * 10^7
= 10 000kg or 1*10^7g

2. Find the number of moles of S

Moles of S = mass of S/relative atomic mass <--- from the periodic table
= 1*10^7/32.07
= 3.12 * 10^4 mol

3. Equate moles to mole ratios

moles of SO2 = moles of S
.'. moles of SO2 = 3.12*10^4

4. Find the volume

Moles of S = volume (the value we want)/volume of 1mole ideal gas at 101.3 kPa and 25C <--- this value is from the data sheet (it's changed for 2005, but for 2004 it was 24.47L)

3.12*10^4 = volume/24.47

rearrange and solve for the answer. voila!

like i said its just a summary of everything in the above posts. how that makes it clearer Alexluby

 

[Alexluby]

Hi, thx for the effort... but i don't get what formula you used for part 4..

Moles of S = volume (the value we want)/volume of 1mole ideal gas at 101.3 kPa and 25C <--- this value is from the data sheet (it's changed for 2005, but for 2004 it was 24.47L)

You didn't use n = CV .... how come urs is like n = V / something :O ??

Looking forward to ur reply... exams later today

Thx in advance!

-- is there like anything on this in the chemistry context book?? if so do u know what page?

 

[brightsea]

sorry for not replying both the trial, alexluby.

the formula i used is one of four (mass, concentration, avo's number and volumes) that i was taught in the prelim course.

chem contexts 2 covers mole calculations involving gas volumes on pg 190-195. it goes over the theory behind it and gives both the equations.

# moles of gas = volume of gas at STP/volume of 1 mole of gas at STP

and

#moles of gas = volume of has at SLC/volume of 1 mole of gas at SLC

STP - standard temp and pressure (0C and 101.3 kPa)
SLC - standard lab conditions (25C and 101.3 kPa)

make sure though you don't memorise the book's values for "volume of 1 mole of gas at STP/SLC" as they've changed. they're always on the data sheet

 

[velox]

Alexluby he just re-arranged the formula