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AS Chemistry- helping each other out!

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A84
8
Original post by C0balt
That could work but the gas produced will have nowhere to go so bung could shoot out lol

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thats what i thought also but then whats the right ans??
Original post by A84
Tip for Hess Law and enthalpy changes

CRFP
Cristiano Ronaldo Football Player

so Combustion = Reactants - Products
and Formation = Products - Reactants


This is taking a risk if you do it this way. I would strongly advise that you just learn the definitions of various enthalpy changes (which you need to know anyway) and get used to creating Hess cycles with them. If you just remember a formulaic way of doing questions you run the risk of being tripped up if they give you any data you're not familiar with (as they have in the past). Hess cycles drawn properly will also sort out any sign issues that again might lose marks

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Reply 2362
Avatar for A84
A84
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Original post by samb1234
This is taking a risk if you do it this way. I would strongly advise that you just learn the definitions of various enthalpy changes (which you need to know anyway) and get used to creating Hess cycles with them. If you just remember a formulaic way of doing questions you run the risk of being tripped up if they give you any data you're not familiar with (as they have in the past). Hess cycles drawn properly will also sort out any sign issues that again might lose marks

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Alright thanks :smile:
bruh the exam is in like 10 days for OCR A Unit 1 :s-smilie: i'm beyond screwed
Original post by samb1234
This is taking a risk if you do it this way. I would strongly advise that you just learn the definitions of various enthalpy changes (which you need to know anyway) and get used to creating Hess cycles with them. If you just remember a formulaic way of doing questions you run the risk of being tripped up if they give you any data you're not familiar with (as they have in the past). Hess cycles drawn properly will also sort out any sign issues that again might lose marks

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that is cool
Original post by shamsaidk
bruh the exam is in like 10 days for OCR A Unit 1 :s-smilie: i'm beyond screwed


What are you struggling with in particular?
Could anyone explain to me why the mols of HCl are used as a basis to calculate mass og ZnCl2 and not the mols of ZnO.
I was thinking that due to the fact there are fewer mols of ZnO, this would be the limiting reactant however the mark scheme uses mols of HCl
chem2.png140.6KB
This is the mark scheme
chem.png175.4KB
Original post by AndrewC19
What are you struggling with in particular?

Have you noticed how in every paper there's a question on acid/base reactions and what not? I'm terrible with them! no matter how many times I cover it my mind just refuses to understand :frown: I think the rest is okayish, i'm also weak with equations, you know when you need to work backwards, eg in water of crystallisation... it sucks :frown:
Original post by VetVikki
Could anyone explain to me why the mols of HCl are used as a basis to calculate mass og ZnCl2 and not the mols of ZnO.
I was thinking that due to the fact there are fewer mols of ZnO, this would be the limiting reactant however the mark scheme uses mols of HCl


Zinc oxide is in excess.
Look at the equation and ratio. It is 1:2=ZnO:h:Cl
So moles of HCl is 0.12 and 0.060mol of ZnO is supposed to react but you have 0.0830

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With water of crystallisation it's just practice and knowing what steps to follow, personally I take this approach:
1) Read the question and underline what information you are given.
2) Calculate the masses of anhydrous, and water. (Mass of hydrated - mass of anhydrous = mass of water).
3) Using this calculate the molar mass of water and the molar mass of anhydrous.
4) Divide the molar mass of water by anhydrous molar mass, to calculate your ratio and therefore you're numbers of waters of crystallisation.

This changes slightly dependant on the nature of the question, but if you use this method it shouldn't be too difficult to workout how to solve it!

With acid/base reactions you just have to remember that the ions switch around (sorta): e.g. HCl + NaOH -> NaCl + H2O.
Just remember the word equation and work from there: Acid + Base -> Salt + Water.

It's just a case of thinking about what you'd expect to form, so for more complex acids and bases. Try a load of practice questions, you'll pick up patterns!
What exam board are you with?
Original post by C0balt
Zinc oxide is in excess.
Look at the equation and ratio. It is 1:2=ZnO:h:Cl
So moles of HCl is 0.12 and 0.060mol of ZnO is supposed to react but you have 0.0830

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Can't believe I didn't see that :redface: thankyou :smile:
Original post by VetVikki
Can't believe I didn't see that :redface: thankyou :smile:


Np:redface:

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Original post by C0balt


Hey would that way of working it out apply to every question that asks which is in excess???
Original post by samwillettsxxx
Hey would that way of working it out apply to every question that asks which is in excess???


Should do
Solid is usually in excess though, because you would know that it is in excess visually (solid left) and when filtering interacted acid will not be let through

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Original post by C0balt
Should do
Solid is usually in excess though, because you would know that it is in excess visually (solid left) and when filtering interacted acid will not be let through

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Thank you :smile:
Original post by samwillettsxxx
Thank you :smile:


Anytime :redface:


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hi can someone help me with this question thanks

methanol starts to have toxic effects when it is present in blood at levels of above 200mg in 1000g
express this in parts per million
Original post by ilovecake123
hi can someone help me with this question thanks

methanol starts to have toxic effects when it is present in blood at levels of above 200mg in 1000g
express this in parts per million


Convert 200mg to g
Divide by 1000
Times by 10^6

Think how you would calculate percentage
Same thing, just different number of zeros

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Original post by C0balt
Convert 200mg to g
Divide by 1000
Times by 10^6

Think how you would calculate percentage
Same thing, just different number of zeros

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how do you convert into grams?
thanks again

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