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A-level Chemistry Revision Squad!

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Original post by Enderbat1999
can someone help me with percentage uncertainty


for uncertainties all u have to know is this :
in the case of addition or subtraction u add the absolute uncertainty ......
in the case of division or multiplication u add up the % uncertainty
u calculate the % uncertainity like this (absolute uncertainty/value*100)
now for eg u have to calculate the percentage uncertainty of a line:
2.59 +/- 0.1
so the % uncertainity will be
0.1/2.59*100 = 3.86%
and the absoute uncertainity will be 0.1 in this case .....
hope that helps ///:h:
On edexcel past papers the number of decimal places they want your answer to be in seems to be constantly changing and is often not stated in the question.. This means I spend a lot of time wondering if I got the marks or not... could anybody explain what the phrase "IGNORE sf except 1 sf" means?

EG, I round my answer to 0.015 and the mark scheme says 0.0149 but to "IGNORE sf except 1 sf"
For the AS AQA new spec, where can I find questions related to the practicals we did?
Original post by alyoan tariq
for uncertainties all u have to know is this :
in the case of addition or subtraction u add the absolute uncertainty ......
in the case of division or multiplication u add up the % uncertainty
u calculate the % uncertainity like this (absolute uncertainty/value*100)
now for eg u have to calculate the percentage uncertainty of a line:
2.59 +/- 0.1
so the % uncertainity will be
0.1/2.59*100 = 3.86%
and the absoute uncertainity will be 0.1 in this case .....
hope that helps ///:h:


Dood thanks that makes sense now thank you
ImageUploadedByStudent Room1463812031.432943.jpg

Would I still get the mark if I drew the arrow like this?


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No has to be on the diagram and one thing why is the arrow that way can you explain
Original post by NimbleNeil
On edexcel past papers the number of decimal places they want your answer to be in seems to be constantly changing and is often not stated in the question.. This means I spend a lot of time wondering if I got the marks or not... could anybody explain what the phrase "IGNORE sf except 1 sf" means?

EG, I round my answer to 0.015 and the mark scheme says 0.0149 but to "IGNORE sf except 1 sf"


Usually, you round your answer according to data significant figures that is given in the question, if the question asked like there is 0.01 mol of Mg and 1.2 mol of Na, the both number of moles is to 2 s.f then you'll have to round your answer to 2 s.f , when the data in the question was given to different s.f, you have to round your answer to at least 3 s.f , and that's why in this case the mark scheme would say, accept any answer except to 1s.f which means if you have rounded your answer up to 1 s.f then it is rejected
(edited 7 years ago)
Original post by Enderbat1999
No has to be on the diagram and one thing why is the arrow that way can you explain


Because the lone pair of electrons electrons is being donated to the N- from the nitrogen.


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For aqa
How do you know when to use phenyl- and when to use -benzene

In the textbook there's benzenecarboxylic acid, benzenecarbaldehyde, and a few other benzene ones then it has phenylamine
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Original post by Bloom77
ImageUploadedByStudent Room1463812031.432943.jpg

Would I still get the mark if I drew the arrow like this?


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I doubt it. Why would you draw it like that?

Original post by olivia7001
For aqa
How do you know when to use phenyl- and when to use -benzene

In the textbook there's benzenecarboxylic acid, benzenecarbaldehyde, and a few other benzene ones then it has phenylamine


If they ask you to name something, use phenyl.
guys can someone put all the equation we need to know with halogens. especially the redox ones with h2so4
why is the ionic equation of a metal carbonate+acid CO3^2- + 2H+ = H2O + CO2 if metal carbonates are solid and in ionic equations you only separate aqueous solutions?
guys what do you think the 8 markers are gonna be like based on those practicals
Can somebody help with this, pretty pretty pretty please?

Which of the following contains the most chloride ions?
A 10cm3 of 3.30X10^-2 moldm^-3 aluminium chloride solution
B 20cm3 of 5.00X10^-2 moldm^-3 calcium chloride solution
C 30cm3 of 3.30X10^-2 moldm^-3 hydrochloric acid
D 40cm3 of 2.50X10^-2 moldm^-3 sodium chloride solution
Original post by angellll
Can somebody help with this, pretty pretty pretty please?

Which of the following contains the most chloride ions?
A 10cm3 of 3.30X10^-2 moldm^-3 aluminium chloride solution
B 20cm3 of 5.00X10^-2 moldm^-3 calcium chloride solution
C 30cm3 of 3.30X10^-2 moldm^-3 hydrochloric acid
D 40cm3 of 2.50X10^-2 moldm^-3 sodium chloride solution


I'm guessing you'll have to find the moles of each one and see which is the highest
So from what I've worked out, you both get 1 mole for B and D
And from the molecular formula, I'll go with B because you get CaCl2 and not NaCl which is 2 moles of chlorine
Is that right?
I'm totally guessing lol


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Original post by Bloom77
I'm guessing you'll have to find the moles of each one and see which is the highest
So from what I've worked out, you both get 1 mole for B and D
And from the molecular formula, I'll go with B because you get CaCl2 and not NaCl which is 2 moles of chlorine
Is that right?
I'm totally guessing lol


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Yes it is, thank you. I got up to the bit where B and D had 1 mole. I just didn't know where to go from there.
Original post by PlayerBB
Usually, you round your answer according to data significant figures that is given in the question, if the question asked like there is 0.01 mol of Mg and 1.2 mol of Na, the both number of moles is to 2 s.f then you'll have to round your answer to 2 s.f , when the data in the question was given to different s.f, you have to round your answer to at least 3 s.f , and that's why in this case the mark scheme would say, accept any answer except to 1s.f which means if you have rounded your answer up to 1 s.f then it is rejected


Thankyou!

SO just to confirm, my answer of 0.15 vs 1.149 is okay because I rounded to 2 sf rather than just 1?
Original post by NimbleNeil
Thankyou!

SO just to confirm, my answer of 0.15 vs 1.149 is okay because I rounded to 2 sf rather than just 1?


Np!

And yeah it is fine!
How many practical questions come up in the exam for paper 1
To calculate empirical formula first work out the number of moles of carbon and hydrogen. You do this by dividing the percentages of each element by the ram. So carbon would be 82.8/12. Then Hydrogen is 17.2/1

Then divide by the smaller decimal to get a 1:x ratio of some sort. Remember when you do that you cannot have a decimal so either round down or up. That will give you the empirical formula. Then look at the ratio between the empirical and molecular formula. It will be an easy whole number.

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