Chemistry: Post your doubts here!

[mehran46]

Okay so the equation is as follows
2SO2 + O2 ---> 2SO3
So we have 2 moles of SO2, 1 mole of O2 and 2 moles of SO2.
We are also given the final reading of O2.
1- Make a table as follows
View attachment 63308
2- Start with filling out the inital and final readings.
Now always remember that THE CHANGE IS ALWAYS WITH RESPECT TO THE NUMBER OF MOLES IN THE EQUATION.
So if you are denoting the change in O2 by 'x' it should be 2x for SO2 as it has 2 moles.
And don't forget to put the '-' sign as they decrease when more product is made.
3- Find the change in O2 by using final - inital = 1 - 0.505 = 0.495.
So if x = 0.495, then 2x = 2 x 0.495.
Now add the value of change in initial (REMEMBER FOR REACTANTS THE READING IS NEGATIVE)
So you will get
Final SO3 = 0 + (+2 x 0.495) = 0.99
Final SO2 = 1 + (-2 x 0.495) = 0.01

Thanks for the continuous help

 

[Holmes]

even my friend had trouble helping me understand that
she had to see me in person in order to explain it :/
in short just imagine a 3D diagram n imagine rotating the groups around clockwise or anticlockwise

helps a bit better if you hold three pens in your hand

Thanks.

 

[MShaheerUddin]

Anybody got AS level chemistry definition list compiled from past papers?

 

[Mstudent]

Why do you multiply the conc. of HCOO- ions by 25? Anyone?

 

[anastasia grey113]

Why do you multiply the conc. of HCOO- ions by 25? Anyone?
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Because they are asking for concentration of SATURATED solution
And the concentration calculated in previous part is of dilute solution
and saturated solution is 50 times saturated so 50 times r the moles of Mg(HCOO-)2
but u see there r 2 moles of HCOO- in 1 mole of Mg(HCOO-)2
so just multiply it by 50/2 = 25

 

[Fareez]

Hi again in the data booklet states

MnO4- + e --> MnO4 2-

MnO4- + 4H +3e --> MnO2 +2H20

Mn04- +8H +5e -->mn 2+ +4h20

What are the difference between all 3 reactions
And when do we use it in equation?

 

[Mstudent]

Guys, will this graph be correct for this question?

 

[Holmes]

Guys, will this graph be correct for this question?
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I'd say no because temperature can't be constant after a sharp rise. Secondly, it keeps changing the concentration which causes rise in temperature.
I guess I made a point.

 

[anastasia grey113]

Guys, will this graph be correct for this question?
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I don't think this is correct since the reaction is endothermic (the positive enthalpy change says it all). So this means that it will 'absorb' heat energy from the water and it's temperature should fall and the greater is the concentration of the reactant, the more energy all of it will need to react so the more the temperature will drop.
In case of exothermic reactions, YES the graph will be this way.
But that is not the case for endothermic reactions. since there is no rapid heat loss in this case.

 

[Mstudent]

Guys, I never understood this point. Why do we always multiply the term inside the bracket by 2 when we square the term. this is an example 4(ii):

 

[Mstudent]

I don't think this is correct since the reaction is endothermic (the positive enthalpy change says it all). So this means that it will 'absorb' heat energy from the water and it's temperature should fall and the greater is the concentration of the reactant, the more energy all of it will need to react so the more the temperature will drop.
In case of exothermic reactions, YES the graph will be this way.
But that is not the case for endothermic reactions. since there is no rapid heat loss in this case.

I don't think so. Cuz that's what the first diagram shows, temp change increases with increase in conc. whether it be endo or exo

 

[anastasia grey113]

Guys, I never understood this point. Why do we always multiply the term inside the bracket by 2 when we square the term. this is an example 4(ii):
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That is only in compounds where one element has an oxidation state of 2+ while the other has 1- or vice versa.
In the above example, there are 2.5x10-3 moles of the salt.
Which also means that there is 1 mole of SO42- ions.
But as you can see in each molecule there r two atoms of Ag1+
so in 1 mole of the salt, there should be 2 moles of Ag1+ right?
So as the concentration of Ag is double the concentration of SO42- and also the salt itself, it has to be multiplied by 2.

 

[anastasia grey113]

I don't think so. Cuz that's what the first diagram shows, temp change increases with increase in conc. whether it be endo or exo

That's temperature CHANGE not the temperature itself.
Let me give u can example.
Let's suppose u add 1 mole of the salt. The temperature drops to 23 degrees so the change is 25-23 = 2
But if u add two moles, it drops to 21 so the CHANGE should be 25 - 21 = 4
so the change increases but the final temperature decreases.

 

[anastasia grey113]

Part c) for this question.

Part e) for the question below


Why are they done differently?
Sources F/M/17/52
M/J/17/51

 

[ErosKuikel]

Can anybody help me with this? I have no idea how to approach this..

 

[ErosKuikel]

Calculate the ph of buffer formed when 10cm3 of 0.1 mol per dm3 NaOH is added to 10cm3of 0.25 mol per dm3 CH3COOH whose pKa is 4.76

 

[anastasia grey113]

Can anybody help me with this? I have no idea how to approach this..

First calculate no. of moles of NaOH, then no. of moles of the acid.
Out of the total moles of acid, some moles will also react with NaOH to make CH3COONa.
CH3COOH + NaOH ---> CH3COONa + H2O
According to the equation above:
No. of moles of acid reacted to makes salt = No. of moles of NaOH = No. of moles o salt formed (since all reactants and products are in 1:1 ratio)
Then find the no. of moles of acid left by subtracting total no. of moles of salt from no. of moles of acid.
Use the following equation.
pH = pKa + log(Moles of salt/Moles of acid)

 

[ErosKuikel]

Yes .thanks a lot

 

[Mstudent]

Part c) for this question.
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Part e) for the question below
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Why are they done differently?
Sources F/M/17/52
M/J/17/51

Could you elaborate? I see no difference in the way both questions are approached

 

[anastasia grey113]

Could you elaborate? I see no difference in the way both questions are approached

Well you see in the 2nd question, the temperature is found by extending another line across the first three points after the sample was added
While in the first one, the already extrapolated line is used rather than extending another line through the first and second point after the sample was added
The question is that in the first question why is the line with the negative gradient (one showing falling temperature) considered to find the temperature rise unlike in the 2nd question in which the line with the positive gradient (one showing rising temperature) used?