Chemistry: Post your doubts here!

[shazmina]

View attachment 51973 help!

may i know the answer pls?

 

[Xylferion]

View attachment 51906

N2O4 <--> 2NO2 at equilibrium.

Initially, there is 1 mol of N2O4. NO2 is yet to be formed.

When N2O4 dissociates, it gives 2 moles of NO2. 1:2
But it says in the question only 50% dissociates.

so 0.5 remains N2O4, and 0.5 dissociates.
This gives 1 mol of NO2 for every o.5 moles of N2O4. 0.5:1

Now with partial pressures there is something called a mole fraction. If you have the mole fraction, and the total pressure, you can find the partial pressure.

The formula is:
Mole fraction * Total pressure = Partial pressure

In the above reaction, there is 1 +0.5 = 1.5 mols in total.
N2O4 is 0.5/1.5 and NO2 is 1/1.5
This gives you 1/3 and 2/3.

Multiply these values by the total pressure, which is 1 in this case, and you get the partial pressures.

Partial pressure for N2O4 is 1/3 atm and 2/3 atm for NO2.

At equilibrium, there are 2 moles of NO2 for every 1 mole of N2O4.
so Kp = ( 2/3 )^2 / (1/3)^1

This gives you 4/3 atm.

Hope that helped ^_^

 

[Xylferion]

View attachment 51973 help!

Density of ice = 1 gcm^-3
Volume of ice being used = 1 cm^3
Mass of ice being used = 1 gcm^-3 * 1 cm^3
Moles of ice being used = 1 g / 18 g since ice is H2O basically.

This gives you 0.0555 moles of ice being used.

1 mole of a gas occupies a volume of 24 dm^3 at 298 K.
0.0555 moles of a gas would occupy a volume of 0.055 * 24 / 1 = 1.332 dm^3 at 298 K.

We want to find out the volume of steam at 596 K.
So if at 298 K the volume is 1.332 dm^-3.
The volume at 596 K would be 596 K * 1.332 / 298 = 2.664dm^-3

The closest answer to this is 2.67 ( There may be some degree of significance missing ), so C is the answer.

Hope that helps!

 

[My Name]

N2O4 <--> 2NO2 at equilibrium.

Initially, there is 1 mol of N2O4. NO2 is yet to be formed.

When N2O4 dissociates, it gives 2 moles of NO2. 1:2
But it says in the question only 50% dissociates.

so 0.5 remains N2O4, and 0.5 dissociates.
This gives 1 mol of NO2 for every o.5 moles of N2O4. 0.5:1

Now with partial pressures there is something called a mole fraction. If you have the mole fraction, and the total pressure, you can find the partial pressure.

The formula is:
Mole fraction * Total pressure = Partial pressure

In the above reaction, there is 1 +0.5 = 1.5 mols in total.
N2O4 is 0.5/1.5 and NO2 is 1/1.5
This gives you 1/3 and 2/3.

Multiply these values by the total pressure, which is 1 in this case, and you get the partial pressures.

Partial pressure for N2O4 is 1/3 atm and 2/3 atm for NO2.

At equilibrium, there are 2 moles of NO2 for every 1 mole of N2O4.
so Kp = ( 2/3 )^2 / (1/3)^1

This gives you 4/3 atm.

Hope that helped ^_^

Yes that helped!
Ty.

 

[shazmina]

http://maxpapers.com/wp-content/uploads/2012/11/9701_s13_qp_42.pdf
Can anyone please explain me Question 1 part b ( iii ) ...
like do i have to draw 2 tangents for both the graphs at time = 0 and find out the initial rate ie the gradient of these 2 lines or is there some other method for finding it??? i dont rele get the mark scheme .. so plz help

 

[shazmina]

Tha value of Kw is 9.55 * 10^-14 at a certain temperature. Calculate the pH of the water at this temperature ...

CAN someone please explain me how to do this ......

 

[Sarosh Jameel]

plz help !

 

[princess Anu]

plz help !

is 6 C,15 B?

 

[princess Anu]

Isn't this question weird? :/
they are asking for isomers of PENTAN-3- ONE
and I don't think there are any isomers of pentane 3 one because in any isomer I make, the place of carbonyl group no longer stays at 3 as stated by the question

 

[Xylferion]

plz help !

For Question 6, You need to write the reaction out.
Al4C3 + NaOH + H2O --------> NaAlO2 + CxHy

Tackle the carbons first, there's 3 carbons on the left, so there should be 3 on the right.
This gives you:
Al4C3 + NaOH + H2O --------> NaAlO2 + 3CHy

There's 4 Al's on the left, so there should be 4 on the right,
Al4C3 + NaOH + H2O --------> 4NaAlO2 + 3CHy

Now balance sodium,
Al4C3 + 4NaOH + H2O --------> 4NaAlO2 + 3CHy

8 oxygen atoms on the right, so there must be 8 in total on the left.
Al4C3 + 4NaOH + 4H2O --------> 4NaAlO2 + 3CHy

12 hydrogen atoms on the left, so there must be 12 on the right, but since there are 3 Hydrocarbon molecules, we can have it as H4.
Al4C3 + 4NaOH + 4H2O --------> 4NaAlO2 + 3CH4

Giving you CH4 as the hydrocarbon, with your answer being A.

---------------------
K2O is dissolved in 250 cm^3 of water.
25 cm^3 of this is titrated with H2SO4 of conc. 2.00 mol/dm^3 needing 15 cm^3.

First write out a reaction between K2O and H2SO4.

K2O + H2SO4 --------> K2SO4 + H2O

The molar ratio for this is 1:1:1:1

First find out the number of moles of H2SO4 using n = cV

n = 2.00 x 15/1000
n = 0.03 moles

1:1 between K2O and H2SO4 so 0.03 moles of K2O.

They want to know the amount in 250 cm^3 of water, while we only used 25 cm^3.

So multiply 0.03 by 10.
You get 0.3 moles of K2O.

Get the Molar Mass of K2O using the data booklet for that year, which is: ( K = 39.1, O = 16.0 )
39.1 x 2 + 16.0 = 94.2

94.2 x 0.3 = 28.26 ---> 28.3 g

Giving you B as your answer.

Hope that helped

 

[Xylferion]

Isn't this question weird? :/
they are asking for isomers of PENTAN-3- ONE
and I don't think there are any isomers of pentane 3 one because in any isomer I make, the place of carbonyl group no longer stays at 3 as stated by the question

How about these?

 

[princess Anu]

How about these?
View attachment 52028
View attachment 52029

These are no longer Pentanones. You've used 6 carbons

 

[Xylferion]

These are no longer Pentanones. You've used 6 carbons

The main chain is still 5 carbons long, those extra CH3 groups are considered methyl.

The first one is 2-methylpentan-3-one.
The second one is 2,2-dimethylpentan-3-one.

Structural Isomerism splits up into:
1) Positional isomerism
2) Functional group isomerism
3) Chain Isomerism

1 and 3 would not work as you want the ketone to stay on the 3rd carbon, and you cannot rearrange the other carbons or branch them since the functional group in question lies on an odd numbered carbon.

Hope that clears it up a bit

 

[Sarosh Jameel]

is 6 C,15 B?

6 A and 15 B

 

[ashcull14]

how is the ans B?

 

[ashcull14]

why isnt it C?

 

[ashcull14]

 

[Xylferion]

View attachment 52033
how is the ans B?

For 1, bromoethane from ethanol is a reaction that requires heating an alcohol under reflux, and then distilling off the halogenoalkane. The alcohol being ethanol here and the halogenoalkane formed being bromotethane.

The very fact that the halogenoalkane has to be distilled can confirm the necessity of having such a set up, since the halogenoalkane can be collected as a distillate in the flask attached to the end of the liebig condenser.

For 2, ethanal forming from ethanol is due to the oxidation of ethanol. This does require for the reaction mixture to be warmed before the oxidation takes place, and in the end you are also required to collect the aldehyde as a distillate.

For 3, 1,2-dibromoethane forming from bromine and ethene is done by bubbling an alkene through a solution of a halogen on the form X2.

So for 1.2-dibromoehane, it would simply have to be bubbled through a solution of bromine at room temperature ( aka no heating required ).

From all of the above options, the only two that require heating and distillation to take place are 1 and 2. Which is option B.

Hope that helped

 

[Xylferion]

View attachment 52035
why isnt it C?

Both phosphorus and sulfur are non-metallic elements that form acidic oxides, and eventually strong acids in water.

As for their electronic configurations,

Phosphorus is 1s2,2s2,2p6,3s2,3p3 ------> each of the 3 electrons are split into 3px, 3py and 3pz. Hence there are no "paired" electrons.

For Sulfur however, the electronic configuration is 1s2,2s2,2p6,3s2,3p4 ---------> There are 4 electrons, First, 3 electrons fill 3px, 3py and 3pz, followed by the 4th electron going into 3px. This means that sulfur does have " paired " 3p electrons.

So for Phosphorus, both statements are correct and for Sulfur, statement 2 is false. Your final answer will be A.

 

[Xylferion]

View attachment 52036 View attachment 52036

N2 + 3H2 <---> 2NH3
Habers process involves an exothermic reaction. An exothermic reaction favours a lower temperature, and would produce yield at a faster rate and overall increase the total yield.

Secondly you need to look at the x-axis, which is pressure. A higher pressure, would favour the side with a lower number of moles. There's 4 moles on the left compared to 2 on the right. This results in the reaction favouring the production of ammonia, since there are fewer moles on the right.

With this in mind, you need to have a graph with a positive gradient since that would mean the yield increases with pressure, as well as a graph where the lower temperature has the higher overall yield.

That graph is A.

Hope that helped