Chemistry: Post your doubts here!

asma tareen · Jan 12, 2014

What does mono basic mean :/??

daredevil · Jan 13, 2014

asma tareen said:
What does mono basic mean :/??

a base which, when it dissociates, releases ONE OH- ion (hydroxyl ion)
e.g. NaOH, CH3COOH, etc.

likewise a monoacidic e.g. HCl, HNO3, etc

there are dibasic : Mg(OH)2 ; Ca(OH)2
diacidic : H2SO4

and so on so forth.... If you dont get it then tell me and I'll explain further

asma tareen · Jan 13, 2014

I got it thnx alot

BreakingBad · Jan 14, 2014

mahabaloch said:
anyone can help me how to make isomers i always get stuck in that?

Isomers of what, exactly?

hassankhan said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w03_qp_4.pdf
Q4 part c anyone??

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.

abruzzi said:
I just can't get my head through this question. Please help me understand how to balance such types of questions.
I'll be grateful for any tips/tricks given.

Edit:
I realized that what i was doing was a waste of time. The right way to do this is to see both the MnO4- equation and the Mn2+ equation from the data booklet, balance the electrons and merge the equations according to reduction and oxidation reactions.

Abdel Moniem said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w07_qp_1.pdf.
Question 9 please

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.

Isaac CKR · Jan 14, 2014

Calculate the pH of the solution formed by mixing 60cm^3 of 0.2 mol dm^-3 of ethanoic acid with 40cm^3 of a 0.10 mol dm^-3 of sodium hydroxide. Ka for ethanoic acid = 1.80 x 10^-5 mol dm^-3 .

Can anyone help me with this? (Calculating the pH during weak acid-strong base titration)

BreakingBad · Jan 14, 2014

Isaac CKR said:
Calculate the pH of the solution formed by mixing 60cm^3 of 0.2 mol dm^-3 of ethanoic acid with 40cm^3 of a 0.10 mol dm^-3 of sodium hydroxide. Ka for ethanoic acid = 1.80 x 10^-5 mol dm^-3 .

Can anyone help me with this? (Calculating the pH during weak acid-strong base titration)

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)

Isaac CKR · Jan 14, 2014

BreakingBad said:
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 · Jan 14, 2014

Isaac CKR said:
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 · Jan 15, 2014

I understood the mol

BreakingBad said:
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.

BreakingBad said:
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 · Jan 15, 2014

BreakingBad said:
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 · Jan 15, 2014

Abdel Moniem said:
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.

Isaac CKR said:
Yup. That is the solution. pH = 4.44

Cool.

mahabaloch · Jan 15, 2014

BreakingBad said:
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 · Jan 15, 2014

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

BreakingBad · Jan 15, 2014

Abdel Moniem said:
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.

mahabaloch said:
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 · Jan 15, 2014

Thank you for your help

Isaac CKR · Jan 16, 2014

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 · Jan 16, 2014

how to solve simple questions involving chromotography ?

BreakingBad · Jan 16, 2014

sitooon said:
how to solve simple questions involving chromotography ?

I think this is O level.

Abdel Moniem · Jan 16, 2014

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 · Jan 17, 2014

Abdel Moniem said:
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

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Chemistry: Post your doubts here!

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