Chemistry: Post your doubts here!

[Isaac CKR]

I have a method in mind. If you can tell me the answer i might be able to help you (considering it matches my answer)

I will be getting the answer and solution from tomorrow's lecture. I'll write back to you tomorrow!

 

[BreakingBad]

I will be getting the answer and solution from tomorrow's lecture. I'll write back to you tomorrow!

Alright. The equation we'll use would probably be:

pH = pKa + log [A-]/[HA]

Still, post back the answer. Thanks.

 

[Abdel Moniem]

I understood the mol

Isomers of what, exactly?


Q4
c) i) G is strongest. Then E. Then F.
ii)
Here's the concept. Carboxylic acids are stronger than all alcohols because the O=C - O - H group has 2 oxygen atoms causing resonance and making it easier for it to lose the H+ ion. Which means that it's easier for it to dissociate in water, hence carboxylic acids are stronger than alcohols (it would be more accurate to call them phenols as their OH group is attached to the ring) which only have an -OH group. The ring in phenol acts as electron donating group making phenols stronger than aliphatic alcohols (not required for this question). The compound F, however, neither has an OH group nor a O=C - O - H group, making it least acidic.
This is the reasoning behind the reactions that take place and the reactions that don't take place when using the compounds above.
Carboxylic acids (including compound G) react with NaOH (forming salt and water) and Na2CO3 (forming salt, water and CO2)
Phenols (including compound E) react with NaOH (salt and water). However, they don't react with Na2CO3 (being less acidic than carboxylic acids).
Compound F will not react with either one as it is the weakest.




In questions like these, it's best to balance the charge first.
LHS manganese charge is -7
RHS manganese charge is +2
We need 9 H+ ions to neutralize the charge.

Now, we have the equation:

MnO4- + ....H2O2 + 9H+ ==> Mn2+ + ....H2O + ....O2

Balance the hydrogen and the the oxygen. We'll get the equation:

MnO4- + H2O2 + 9H+ ==> Mn2+ + 11/2H2O + 1/4O2

(I have no idea what i'm doing anymore)
Anyways, i'm not really sure if that's the correct answer so, rely on my concept only if the answer is by any chance correct


Moles of metallic salt = vol. * conc. = 5*10^-3
Moles of sodium sulphite = vol. * conc. = 2.5*10^-3

Mole ratio = 1:2
Hence electron ratio = 1:2
The sodium sulphite ion gives 2 electrons. Using the ratio, we can understand that 1 mole of metallic salt would receive 1 electron.
Initially, it was +3. Add an electron and we get +2. Answer is B.

Alright. The equation we'll use would probably be:

pH = pKa + log [A-]/[HA]

Still, post back the answer. Thanks.

I understood the mole ratio part but how is it initially 3 electrons and then you simply say give it one more electron to be a +2

 

[Isaac CKR]

Alright. The equation we'll use would probably be:

pH = pKa + log [A-]/[HA]

Still, post back the answer. Thanks.

Yup. That is the solution. pH = 4.44

 

[BreakingBad]

I understood the mol

I understood the mole ratio part but how is it initially 3 electrons and then you simply say give it one more electron to be a +2

The initial oxidation state is +3. As in it has lost 3 electrons. Since the mole ratio is 1:2. 2 moles of sodium sulphite are required to react with 1 mole of metallic salt. Therefore, 2 moles of electrons released by the sodium sulphite will count as 1 mole received by the metallic salt. +3 oxidation state will get the 1 mole of electron and hence, by gaining an electron, become +2.

Yup. That is the solution. pH = 4.44

Cool.

 

[mahabaloch]

Isomers of what, exactly?


like any isomers.. When i do the past paper questions of isomers i always get stuck in that questions

 

[Abdel Moniem]

Sodium sulphite lost 2 electron it's oxidised and the metallic salt lost 3 electrons. Could you explain in equations please breakingbad

 

[BreakingBad]

Sodium sulphite lost 2 electron it's oxidised and the metallic salt lost 3 electrons. Could you explain in equations please breakingbad

No. The METAL in the salt has an oxidation of +3. During the reaction the salt has to react with sodium sulphite and gain the electrons lost by the sulphite during the reaction. Since one mole of salt reacts with 2 moles sodium sulphite, one electron will be gained for every 2 released. The metal which previously had an oxidation state of +3 would gain that electron and become +2. I really can't explain using equations because we don't have any identity for the metal salt.

like any isomers.. When i do the past paper questions of isomers i always get stuck in that questions

You could look for notes online to understand the concept. Finding isomers is not that hard. Or you could copy a few of those questions and we might be able to help you.

 

[Abdel Moniem]

Thank you for your help

 

[Isaac CKR]

The Ksp of calcium sulfate, CaSO4, is 9.1 x 10^-6 mol^2 dm^-6. What is the molar concentration of CaSO4 in a saturated solution?

 

[sitooon]

how to solve simple questions involving chromotography ?

 

[BreakingBad]

how to solve simple questions involving chromotography ?

I think this is O level.

 

[Abdel Moniem]

Two equilibria are shown below.

reaction I 2X2(g) + Y2(g) gives 2X2Y(g)
reaction II X2Y(g) gives X2(g) + 1/2Y2(g)

The numerical value of Kc for reaction I is 2.

Under the same conditions, what is the numerical value of Kc for reaction II?

 

[ZaqZainab]

Two equilibria are shown below.

reaction I 2X2(g) + Y2(g) gives 2X2Y(g)
reaction II X2Y(g) gives X2(g) + 1/2Y2(g)

The numerical value of Kc for reaction I is 2.

Under the same conditions, what is the numerical value of Kc for reaction II?

I got 1/root of 2

 

[williamcwz]

During the bromination of methane, the free radical •3
CH is generated and a possible terminating
step of this reaction is the formation of C2H6 by the combination of two free radicals.
What could be produced in a terminating step during the bromination of propane?
CH3

1 CH3CH2CH2CHCH3
CH3 CH3
2 CH3CHCHCH3
CH3
3 CH3CH2CHCH2CH3

 

[Abdel Moniem]

June 2007 question 34 and with explanation please to fully understand

 

[BreakingBad]

During the bromination of methane, the free radical •3
CH is generated and a possible terminating
step of this reaction is the formation of C2H6 by the combination of two free radicals.
What could be produced in a terminating step during the bromination of propane?
CH3

1 CH3CH2CH2CHCH3
CH3 CH3
2 CH3CHCHCH3
CH3
3 CH3CH2CHCH2CH3

Please check if you have the right options for this question because the product is supposed to be an alkane or a bromoalkane and none of the options given have the proper number of carbon and hydrogen atoms for it to be an alkane.

June 2007 question 34 and with explanation please to fully understand

1 equilibrium mixture N2F4 <=>2NF2
Enthalpy change is positive which means that the forward reaction is endothermic. When temperature is increased it favors the endothermic reaction and therefore the equilibrium will shift to the right. More moles (since ratio is 1:2) will be produced taking up greater volume and so, the mercury on LHS will be pushed downward and mercury on RHS will rise.

2 equilibrium mixture CH3NC <=>CH3CN
It won't bring about a change as the moles of the gas are same on both sides.

3 nitrogen
When the temperature rises, pressure will rise on both sides equally and so, no change will take place in mercury level.

Since only 1 is right, answer is D

 

[Abdel Moniem]

which ions are present in a solution of ethanol in an excess of concentrated sulphuric acid?
1) CH3CH2O^-1
2) CH3CH2^+OH2
3) HSO4^-1

 

[pikachu]

Can anyone explain this question?

 

[Isaac CKR]

What is the pH in a saturated solution of Ca(OH)2? Ksp = 5.5 x 10^-6 mol^3 dm^-9 for Ca(OH)2

Answer : pH = 12.35

I keep getting pH = 12.05 with my method.

Ksp = [Ca^2+][OH^-]^2

5.5 x 10^-6 = (x) (2x)^2

4x^3 = 5.5 x 10^-6

x = 0.0111 mol dm^-3

pOH = -log [0.0111]
pOH = 1.95

pH = 14 - 1.95 = 12.05. <----- X, answer is 12.35. WHY?