A-Level Chemistry Help

Hi guys, I am a bit confused on how you rearrange the Kc equation to find SO3 concentration as it contains concentrations that are squared. I have attached the question - from 2017 OCR A P1. If anyone could take the time to explain it, I'd really appreciate it! Thanks
2017 Q.png

(edited 2 months ago)

Reply 1

basilpl4nt

so. I'm guessing you know that in this expression, K_c=[SO₃]²/[O₂][SO₂]²
units = dm³mol^-1 (as stated in the question... my teacher gave us this question about 5 times during my A-levels because he found it hilarious that people still got this bit wrong)

anyways. 2 MARKS! in the bag, before you have to worry about rearranging.

so if we plug in the values from the question,

3.045x10⁴=x²/((0.054/0.4)²x(0.027/0.4))
(I divided the mol values by 0.4 because we are told the gas is compressed to 400cm³, a.k.a. 0.4dm³, and we always use concentration for K_c. which you probably know but I always forget. if you type things into the calculator wrong often like me, work these out NOW for ecf marks, to give 3.045x10⁴=x²/(0.135²x0.0675)
now to rearrange. take that whole lump (0.135²x0.0675) at the bottom of the equation on the RHS, and times both sides by that. you'll have 3.045x10⁴x0.135²x0.0675=x². (in case you're wondering where the brackets went, we didn't need them because everything is just being multiplied anyways)

now dealing with the square is simple. just square root both sides to give √(3.045x10⁴x0.135²x0.0675)=x
type that into your calculator. you'll get something along the lines of 6.12039291018. 3 marks!

now, multiply that by 0.4dm³ (because the value we worked out is a concentration, and they asked for moles) to give:
2.45mol!!!! 4 marks, plus the 2 from part a.
I hope this was not too patronising. i found this quite difficult at a-level, so it's how i would've explained it to myself, if that makes sense.

Reply 2

User_F1

Original post by basilpl4nt

so. I'm guessing you know that in this expression, K_c=[SO₃]²/[O₂][SO₂]²
units = dm³mol^-1 (as stated in the question... my teacher gave us this question about 5 times during my A-levels because he found it hilarious that people still got this bit wrong)
anyways. 2 MARKS! in the bag, before you have to worry about rearranging.
so if we plug in the values from the question,
3.045x10⁴=x²/((0.054/0.4)²x(0.027/0.4))
(I divided the mol values by 0.4 because we are told the gas is compressed to 400cm³, a.k.a. 0.4dm³, and we always use concentration for K_c. which you probably know but I always forget. if you type things into the calculator wrong often like me, work these out NOW for ecf marks, to give 3.045x10⁴=x²/(0.135²x0.0675)
now to rearrange. take that whole lump (0.135²x0.0675) at the bottom of the equation on the RHS, and times both sides by that. you'll have 3.045x10⁴x0.135²x0.0675=x². (in case you're wondering where the brackets went, we didn't need them because everything is just being multiplied anyways)
now dealing with the square is simple. just square root both sides to give √(3.045x10⁴x0.135²x0.0675)=x
type that into your calculator. you'll get something along the lines of 6.12039291018. 3 marks!
now, multiply that by 0.4dm³ (because the value we worked out is a concentration, and they asked for moles) to give:
2.45mol!!!! 4 marks, plus the 2 from part a.
I hope this was not too patronising. i found this quite difficult at a-level, so it's how i would've explained it to myself, if that makes sense.

Ahh ok! Yes! You broke that down for me so well - thank you!