Chemistry: Post your doubts here!

[Metanoia]

Thanks dear for your answer, but I was specifically needing help for no. 2, I don't know how to make [Pb2+] equal to [Cl-]. I hope you understand when the power is 1 for the concentration I can calculate it easily but I am having problem when there is a power greater than 1...........waiting for your response

Let concentration of eqm concentration of Pb2+ be x

[Cl-] = 2x

Ksp = [Pb2+][Cl-]^2
= (x) (2x)^2
= (x)(4x^2)
= 4x^3

 

[Rectified Spirit]

Thanks dear for the help.....

 

[Rectified Spirit]

Hi can pls help me with this problem. 9701/41/O/N/2012 qstn 1 b I. Why cannot we choose any equation for MnO4- to solve the problem....I chosed the one with E০ 1.67V, but the MS worked this problem out with the one having a value of 1.52V....... I have attached the pics........

 

[Metanoia]

Hi can pls help me with this problem. 9701/41/O/N/2012 qstn 1 b I. Why cannot we choose any equation for MnO4- to solve the problem....I chosed the one with E০ 1.67V, but the MS worked this problem out with the one having a value of 1.52V....... I have attached the pics........View attachment 57150 View attachment 57151

Short answer:

1.67V is for neutral/mildly alkaline conditions
1.52V is for acidic conditions.

In case you are wondering, the 0.56V half equation is not commonly needed as the MnO42- is unstable and tends to undergo disproportionation.

 

[Wes Allen]

(b) A mixture containing 0.50 mol of CO2, 0.50 mol of H2, 0.20 mol of CO and 0.20 mol of H2O was placed in a 1.0 dm3 flask and allowed to come to equilibrium at 1200 K. Calculate the amount, in moles, of each substance present in the equilibrium
mixture at 1200 K.

Can someone please help me with this question?

 

[yash gujarati]

chemistry MCQ may/june 2015 varient 2 question 2

The shell of a chicken’s egg makes up 5% of the mass of an average egg. An average egg has a mass of 50 g.
Assume the egg shell is pure calcium carbonate.
How many complete chicken’s egg shells would be needed to neutralise 50 cm3 of 2.0 mol dm–3 ethanoic acid?
A 1 B 2 C 3 D 4

I assumed its D buts its B and how?

 

[The Sarcastic Retard]

(b) A mixture containing 0.50 mol of CO2, 0.50 mol of H2, 0.20 mol of CO and 0.20 mol of H2O was placed in a 1.0 dm3 flask and allowed to come to equilibrium at 1200 K. Calculate the amount, in moles, of each substance present in the equilibrium
mixture at 1200 K.

Can someone please help me with this question?

For this we need the value of Kc at 1200K temperature. It will be mentioned somewhere near the equation. Use that to solve this problem, if u dont know how to solve it, just temme the Kc value, i will explain you... Also the balanced equation.

 

[psychiatrist]

chemistry MCQ may/june 2015 varient 2 question 2

The shell of a chicken’s egg makes up 5% of the mass of an average egg. An average egg has a mass of 50 g.
Assume the egg shell is pure calcium carbonate.
How many complete chicken’s egg shells would be needed to neutralise 50 cm3 of 2.0 mol dm–3 ethanoic acid?
A 1 B 2 C 3 D 4

I assumed its D buts its B and how?

This is what I did in my exams,
CaCO3 + 2CH3COOH ----> (CH3COO)2Ca + CO2 + H2O
The shell of a chicken’s egg makes up 5% of the mass of an average egg and average mass is 50g
moles of egg shell = 5% of 50/100 = 0.025mol
moles of ethanoic acid = 50*10^-3 * 2 = 0.1. According to stoichiometry, 0.1/2 = 0.05mol
We are asked How many complete chicken’s egg shells would be needed to neutralise 50 cm3 of 2.0 mol dm–3 ethanoic acid?
So it will be moles of CaCO3 / moles of CH3COOH = 2

 

[Wes Allen]

For this we need the value of Kc at 1200K temperature. It will be mentioned somewhere near the equation. Use that to solve this problem, if u dont know how to solve it, just temme the Kc value, i will explain you... Also the balanced equation.

Value of Kc is 1.44
The reaction is CO2 + H2 ----> CO + H2O

 

[The Sarcastic Retard]

Value of Kc is 1.44
The reaction is CO2 + H2 ----> CO + H2O

Kc = [CO][H2O]/[CO2][H2]
1.44 = (0.2+x)^2 / (0.5-x)^2
sqrt(1.44) * (0.5-x) = 0.2+x
0.6 - 1.2x = 0.2 + x
0.4 = 2.2x
x = 0.18
Equilibrium moles of CO and H2O= 0.2 + 0.18 = 0.38 and of CO2 and H2 = 0.32

 

[chocolatelover]

I know how to make compounds D,E and F. But what is the point of knowing that when I don't even know how to make A, B and especially C.
Yeah, so I need to know how to make compounds A, B and C.

 

[The Sarcastic Retard]

View attachment 57169
I know how to make compounds D,E and F. But what is the point of knowing when I don't even know how to make A, B and especially C.
Yeah, so I need to know how to make compounds A, B and C.

Esterification in A and B and elimination in C

 

[The Sarcastic Retard]

View attachment 57169
I know how to make compounds D,E and F. But what is the point of knowing that when I don't even know how to make A, B and especially C.
Yeah, so I need to know how to make compounds A, B and C.

 

[Serial_Ripper]

Hello, can you please tell me if it's possible to finish paper 5 in a week? given that at the beginning of said week I never opened any paper and at the end of the same week is the actual exam i.e. 30 October - 5 November?

PLEASE REPLY!!

 

[nehaoscar]

How is C red and D blue?
Looking at the peaks you get C blue and D red :S
How to do this one???

 

[nehaoscar]

Hello, can you please tell me if it's possible to finish paper 5 in a week? given that at the beginning of said week I never opened any paper and at the end of the same week is the actual exam i.e. 30 October - 5 November?

P5 is not exactly difficult, you just need to know how to plan experiment and the basic techniques. I would advise to revise earlier but in the worst case I believe yes it can be done.

 

[Serial_Ripper]

P5 is not exactly difficult, you just need to know how to plan experiment and the basic techniques. I would advise to revise earlier but in the worst case I believe yes it can be done.

THANK YOU!!

 

[Liv2Stdy]

View attachment 57174
How is C red and D blue?
Looking at the peaks you get C blue and D red :S
How to do this one???

Its not like that. The peak of c is at 450 nm which means that it absorbs all of blue and gives out its complementary color, red and the same thing happens at peak of D, it absorbs all of rwd and gives out the complementary color which is blue.

 

[yash731]

The products obtained by cracking an alkane, X, are methane, ethene and propene. The mole fraction of ethene in the products is 0.5. What is the identity of X?
A C6H14 B C8H18 C C9H20 D C11H24

and

40 An organic compound, X, will react with an excess of calcium metal to produce a salt with the empirical formula CaC4H6O4. What could be the identity of X?
1 ethanoic acid 2 butanedioic acid 3 methylpropanedioic acid

May june 2011 mcq paper-12

 

[The Sarcastic Retard]

The products obtained by cracking an alkane, X, are methane, ethene and propene. The mole fraction of ethene in the products is 0.5. What is the identity of X?
A C6H14 B C8H18 C C9H20 D C11H24

and

40 An organic compound, X, will react with an excess of calcium metal to produce a salt with the empirical formula CaC4H6O4. What could be the identity of X?
1 ethanoic acid 2 butanedioic acid 3 methylpropanedioic acid

May june 2011 mcq paper-12

1)
Mole fraction = [No. of mole of molecule]/[No. of total moles]
Let mole fraction = X
X(C2H4) = 0.5
So we can assume no. of moles of CH4 and C3H6 to be 1 and C2H4 to be 2 so that 2/(1+2+1) = 0.5 mol
So we have products as CH4 + 2C2H4 + C3H6
Total of 8 C and 18 H
Therefore answer is C8H18

2)
1.CH3COOH + Ca --------> Ca(CH3COO)2 = CaC4H6O4 (correct)
2.HOOCCH2CH2COOH + Ca ------------> CaC4H4O4 (Incorrect)
3. I am not sure about this because as per my knowledge the name should be 2-methypropanedioc acid.
HOOCCH(CH3)COOH + Ca ------> CaC4H4O4 (Incorrect)