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OCR Chemistry A Exam Thread (Breadth - May 27 2016 and Depth - June 10 2016)

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URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.
Original post by asinghj
Electronegativity is the attraction of a atom to get the shared pair of electrons, for which you need to know how it changes (increases going up and increases going from left to right)

Periodicity is the repeating trends across each period of the periodic table (you need to know first ionisation energy increases across a period, the trend in melting and boiling point and atomic radius)

Formation of pi bonds I have no clue so help?


Thanks.

I just had look and pi bonds are formed after a sigma bond has been formed and is formed by the p-orbitals overlapping above and below the carbon atoms
Reply 802
Avatar for dj2007
dj2007
1
Original post by maxross
URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.


m3 and pa
Original post by maxross
URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.


yh the ones you mention are right
Original post by 4nonymous
yh the ones you mention are right


Thank you guys. I though they were but some questions i have seen use dm3 and KPa?? oh well, in the exam today ill use m3 and Pa.

Thank you so much
Original post by maxross
URGENT QUESTION!!!
What are the SI units to use in the Pv=nRT equation?? i have seen so many different websites and textbooks say different things. aaaaargh. I thought it was
P= Pa (not KPa)
v= m3 (not dm3)
n is obviously just n, same as R and T is definitely in Kelvin.
Please someone help. Its confusing me and annoying me because the equation isn't hard.


Pressure is Pa (even though Pa is technically not SI units, it's derived from Jm^-3)
Volume is m^3 (that is definitely from SI units)
Original post by maxross
Thank you guys. I though they were but some questions i have seen use dm3 and KPa?? oh well, in the exam today ill use m3 and Pa.

Thank you so much


Don't want to confuse you so you might not want to read this:
If you use kPa that is derived from Jdm^-3 so for volume you would use dm^3. it's technically not wrong.. But stick to Pa and m^3
Original post by asinghj
Pressure is Pa (even though Pa is technically not SI units, it's derived from Jm^-3)
Volume is m^3 (that is definitely from SI units)


QUESTION :

1.68g noble gas occupies volume of 500cm3 at RTP (298k and 100KPa). Identify noble gas.

can you do this for me and put your workings please???
Hey you know for the enthalpy calculations? When you're working out a question using formation values in the table is it product - reactants, and combustion values reactants - products and change of reaction/ bond enthalpy products - reactants??? I always get confused on which one is which
Original post by maxross
QUESTION :

1.68g noble gas occupies volume of 500cm3 at RTP (298k and 100KPa). Identify noble gas.

can you do this for me and put your workings please???


Ok that is using RTP, which is not 298K and 100kPa (that is STP)

At RTP, n = volume / molar volume
So n = 500 / 24000 = 0.0208 mol
Also, n= mass / molar mass
So molar mass = mass / n
So Mr = 1.68 / 0.0208 = 80.77

Since it's a noble gas, it must be krypton (closest molar mass)

This is how I would do it but for some reason it doesn't seem right, anyone?
(edited 7 years ago)
Original post by snickercell
Hey you know for the enthalpy calculations? When you're working out a question using formation values in the table is it product - reactants, and combustion values reactants - products and change of reaction/ bond enthalpy products - reactants??? I always get confused on which one is which




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1465548497957.jpg33.2KB
Original post by maxross
QUESTION :

1.68g noble gas occupies volume of 500cm3 at RTP (298k and 100KPa). Identify noble gas.

can you do this for me and put your workings please???


500/24000 = 0.0208 mol (3sf)

1.68 / 0.0208 = 80.8 g mol-1

Kr due to different isotopic abundances
(edited 7 years ago)
Reply 812
Avatar for MissPR
MissPR
1
Could someone explain why pent-1-ene does not show stereoisomerism please?
Original post by asinghj
Ok that is using RTP, which is not 298K and 100kPa (that is STP)

At RTP, n = volume / molar volume
So n = 500 / 24000 = 0.0208 mol
Also, n= mass / molar mass
So molar mass = mass / n
So Mr = 1.68 / 0.0208 = 80.77

Since it's a noble gas, it must be krypton (closest molar mass)

This is how I would do it but for some reason it doesn't seem right, anyone?


that is correct answer in book. but they have worked it out using Pv=nRT
Original post by maxross
that is correct answer in book. but they have worked it out using Pv=nRT


1 mol of gas occupies 24.0 dm3 at RTP (approximately)
Original post by MissPR
Could someone explain why pent-1-ene does not show stereoisomerism please?


On one of the carbons in C=C there are the same groups (2 hydrogens) and therefore, there is no priority group
Reply 816
Avatar for MissPR
MissPR
1
Original post by asinghj
On one of the carbons in C=C there are the same groups (2 hydrogens) and therefore, there is no priority group


Thank youu :smile:
Original post by maxross
that is correct answer in book. but they have worked it out using Pv=nRT


From what I've learnt, for gases at RTP, use n= volume / molar volume
Original post by asinghj
From what I've learnt, for gases at RTP, use n= volume / molar volume


nice.

what you recon'll come up today???
Original post by snickercell
Hey you know for the enthalpy calculations? When you're working out a question using formation values in the table is it product - reactants, and combustion values reactants - products and change of reaction/ bond enthalpy products - reactants??? I always get confused on which one is which


I think its always reactants - products

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