Chemistry: Post your doubts here!

[Sarosh Jameel]

Thanks man

When the question states that Y gives a ppt "with" aqueous silver nitrate, you should immediately be able to remember that this is a test for halide ions. Which are the group 7 elements consisting of Cl, Br and I.

The second part of the question is also important, since it can help us eliminate an option.

All 3 compounds, give a ppt with aqueous silver nitrate.
Cl- is soluble in aqueous ammonia.
Br- is slightly soluble in "dilute" ammonia, but soluble in "concentrated" ammonia.
I- is insoluble in both, dilute and concentrated.

Since Iodine disagrees with statement two, which is that it dissolves in conc. ammonia when it clearly does not, we can eliminate option 3.

Both option 1 and 2 have halide ions that are soluble in concentrated ammonia, so they are correct.

Your answer should be B, since that states that both option 1 and 2 are correct.

Hope that helped!

Thanks man !

 

[Xylferion]

Which isomer of C4 H10 O forms three alkenes on dehydration ?
A . butan-1-ol
B . butan-2-ol
C. 2-methylpropan-1-ol
D 2-methylpropan-2-ol

Primary alcohols, upon dehydration, will always give 1 product.
Secondary or tertiary alcohols can give 1 or more products, as a result of this you can also have cis/trans products. Each counting as one product.

Knowing this you can safely eliminate option A and C. Now you're left with B and D, which is butan-2-ol and 2-methylpropan-2-ol.

At this point I would recommend drawing out the displayed formula of both, so you are able to see where the double bond can be made when the -OH group is removed.
Since dehydration is going from an alcohol -------> an alkene.

For 2-methylpropan-2-ol, There's 3 carbons attached to the carbon holding the -OH group. Which means it's a tertiary alcohol. Another thing to note is that you can only form 1 double bond with any of the 3 carbons, meaning you can only have 1 product in this case.

For butan-2-ol, the second carbon is holding the -OH group. There are 4 carbons in the chain, if the -OH group is removed, the 2nd carbon can only form a double bond with the 1st and 3rd carbon ( its neighbours ). This is where the cis/trans isomerism thingy comes to the rescue, when the double bond is between the 2nd and 3rd carbon, you can now have cis and trans isomerism across the double bond.



This gives you 3 products from butan-2-ol, and so your answer is B.

Hope that helps! If it didn't make sense, I can draw out the other products for you.

 

[The Chill Master]

Errr... This question is quite tricky to explain partially because of the fact that the question is based off of the syllabus back in 2009.

Anyways, the process of forming the cyclic compound from J, is known as cyclic esterification. Cyclic esters are actually called lactones.

The process is as follows, if you have a hydroxy-carboxylic acid ( Hydroxypropanoic acid in this question ), you can have it undergo an "intra"molecular reaction.

Normally during any esterification process, you want a carboxylic acid, alcohol and some heat. However in cyclic esterification, you do not need the presence of an alcohol. You can make do with simply having a hydroxy carboxylic acid and some heat.

That is where the term " intra " comes into play, they have the hydroxycarboxylic acid react within itself to form an ester that has the ability to form a cyclic compound.

There's some advanced theory that involves the inclusion of a dynamic equilibrium, hence the requirement of a catalyst is necessary, more specifically an acid catalyst.

In this question, you have your hydroxycarboxylic acid ( hydroxypropanoic acid ) and an acid catalyst ( Conc. sulfuric acid. ). You're also told that the molecular formula is C6H8O4.

At the start of the question, it mentions that J has 3 carbons. If the molecular formula of L has 6 carbons, that means two molecules of J have been used, and so 2 ester groups will form.

With the two Ester groups, you can have them attach in the shape of a ring ( forming a cyclic compound ), both of them having the formula RCOO-, the diagram in the marking scheme is a good example of how they attach.

View attachment 52069 The red part is one ester group. You can tell since R is C2H4 and then you have COO.

I would just recommend knowing what is required to make a cyclic ester, and the basic ester group notation. The rest is far too complicated for AS level, since lactones are technically part of the A2 syllabus.

Hope that helped

Oh Thanks a dozen...............
It really helped a lot

O/N/13/21 Q 5 part (f) can you please explain????
And when it comes to these types of questions what sort of clues should we take from question to help us draw the structure?

One again Thanks

 

[Xylferion]

Oh Thanks a dozen...............
It really helped a lot

O/N/13/21 Q 5 part (f) can you please explain????
And when it comes to these types of questions what sort of clues should we take from question to help us draw the structure?

One again Thanks

There's no part f :/

Generally you want to know how many carbons, hydrogens, oxygens if any, are there. Then you want to know what kind of functional groups are present. Accordingly you'll have to arrange the functional groups and the length of the longest chain in order to get what you need.

 

[3l_general]

question 4 plzzzzzzzz ,,,

 

[Xylferion]

question 4 plzzzzzzzz ,,,

View attachment 52085

For a fixed quantity of gas, that is the amount of gas present does not increase ( add gas ) or decrease ( gas escapes ), ( A closed valve indicates that nothing is escaping ) we can use the following relation:

p1V1/T1 =p2V2/T2

1 x 10^ 5 * V / 293 Kelvin = final pressure * 4V / 373 Kelvin

V is the volume before the valve is opened. 4V is the total volume after some gas is evacuated. That is 3V for N and 1V for M.

341.3 V = final pressure * 4V / 373 K

Final pressure = 373 K * 341.3 V / 4V
Final pressure = 31826.225 pascals -------> 3.18 x 10^4 Pascals.

Hope that helped!

 

[Xaptor16]

Total removal of the pollutant sulfur dioxide, SO2, is difficult. The quantities emitted from furnace chimneys can be lowered by using desulfurisation plants. The gases are reacted with calcium hydroxide to remove the SO2. What is the main product formed initially?
A Ca(HSO4)2
B CaS
C CaSO3
D CaSO4
help! i don't get how the answer is C? how do you know its not D?
19 Which oxide does not react with cold dilute sodium hydroxide to produce a salt?
A Al 2O3
B P4O10
C SO2
D SiO2
the asnwer is D... i dont understand how!

 

[Xaptor16]

The ester CH3CH2CH2CO2CH3 is responsible for the aroma of apples. When this ester is hydrolysed by acid in the stomach, what is the empirical formula of the organic acid produced?
A CH2O
B CH4O
C C2H4O
D C3H6O2
can someone please explain how its C? wouldnt it be A?

 

[Lola_sweet]

The ester CH3CH2CH2CO2CH3 is responsible for the aroma of apples. When this ester is hydrolysed by acid in the stomach, what is the empirical formula of the organic acid produced?
A CH2O
B CH4O
C C2H4O
D C3H6O2
can someone please explain how its C? wouldnt it be A?

If u draw the structure of A u will see it cant exsist carbon will still ve one empty bond pair or branch left thus it has to be C
I think thts right :/

 

[Nicolle L]

Magnesium nitrate, Mg(NO3)2, will decompose when heated to give a white solid and a mixture of gases. One of the gases released is oxygen. 29.7g of anhydrous magnesium nitrate is heated until no further reaction takes place. What mass of oxygen is produced?
A 3.2g
B 6.4g
C 12.8g
D 19.2g

 

[Xaptor16]

If u draw the structure of A u will see it cant exsist carbon will still ve one empty bond pair or branch left thus it has to be C
I think thts right :/

yes i know, but its the empirical formula we're talking about, it wouldnt make sense to draw the structure out as its not the molecular formula...

 

[Xylferion]

Magnesium nitrate, Mg(NO3)2, will decompose when heated to give a white solid and a mixture of gases. One of the gases released is oxygen. 29.7g of anhydrous magnesium nitrate is heated until no further reaction takes place. What mass of oxygen is produced?
A 3.2g
B 6.4g
C 12.8g
D 19.2g

2Mg(NO3)2 ------> 2MgO + 4NO2 + O2

Mg(NO3)2 = 148 g
Moles of Mg(NO3)2 = 29.7 / 148 =0.201

Ratio is 2 : 1
So 0.1005 moles of O2 formed.
0.1005 moles * 32 g ( Molar mass of O2) = 3.216 g , The answer is A.

Hope that helped

 

[Xylferion]

The ester CH3CH2CH2CO2CH3 is responsible for the aroma of apples. When this ester is hydrolysed by acid in the stomach, what is the empirical formula of the organic acid produced?
A CH2O
B CH4O
C C2H4O
D C3H6O2
can someone please explain how its C? wouldnt it be A?

CH3CH2CH2CO2CH3 undergoes acid hydrolysis in the stomach under the presence of water, with HCl being the acid catalyst of the hydrolysis.

[CH3CH2CH2CO2] comes from the carboxylic acid and CH3 comes from the alcohol. During esterification, OH is taken from the alcohol and H is taken from -COOH in the carboxylic acid.

When hydrolysis takes place, the OH and H are returned to their respective compounds.

So you end up with CH3CH2CH2CO2H for the carboxylic acid and CH3OH for the alcohol.

They're asking for the empirical formula of the acid, so first find out how many carbons, hydrogens and oxygens [CH3CH2CH2CO2H] has.

That is 4 carbons, 8 Hydrogens and 2 oxygens.

C4H8O2 ------> C2H4O, which is why the answer is C.

 

[Xylferion]

Total removal of the pollutant sulfur dioxide, SO2, is difficult. The quantities emitted from furnace chimneys can be lowered by using desulfurisation plants. The gases are reacted with calcium hydroxide to remove the SO2. What is the main product formed initially?
A Ca(HSO4)2
B CaS
C CaSO3
D CaSO4
help! i don't get how the answer is C? how do you know its not D?
19 Which oxide does not react with cold dilute sodium hydroxide to produce a salt?
A Al 2O3
B P4O10
C SO2
D SiO2
the asnwer is D... i dont understand how!

For the first one,
Ca(OH)2 + SO2 -------> CaSO3 + H2O
Ca combines with SO2 ---> CaSO2 and then the extra Oxygen atom comes from (OH)2. The other product is water.

The process mentioned above is known as flue-gas desulfurization. Which is dealing with the waste gases that contain SO2. To be more specific, they use wet scrubbing to get rid of the SO2 in the form of CaSO3 which is solid.

CaSO4 can then be formed by oxidizing CaSO3 so that's probably why CaSO3 takes precedence over CaSO4, in this specific reaction. Though generally it is CaSO3 that you want to have form from the reaction between SO2 and Limestone/lime.

----------------------------------

All of the oxides listed are of period 3 elements. I suggest you have a look through the syllabus content and find out what exactly you must know about period 3 elements besides their trends. You are required to know how their oxides react.

All of the above oxides can react with NaOH. Al2O3 is amphoteric so it can react with both acids and bases.

P4O10, SO2 and SiO2 form acidic oxides that can react with bases. The exception here is that SiO2 only reacts with Hot and concentrated NaOH as opposed to the other 3.

Since the question mentions those that "do not" react with cold dilute NaOH, you can with absolute certainty rule out SiO2 and have that as your final answer.

As for understanding why it does not, I recommend looking into how to determine strong and weak acids/bases. So then you can understand whether dilute/concentrated substances can have an effect on the outcome.

For SiO2, from what I know, its oxide is very weakly acidic and can only be neutralized by strong bases such as NaOH. So a dilute NaOH would not have as much of an effect on SiO2 as a hot and concentrated one would.

Hope that made sense!

 

[The Chill Master]

There's no part f :/

Generally you want to know how many carbons, hydrogens, oxygens if any, are there. Then you want to know what kind of functional groups are present. Accordingly you'll have to arrange the functional groups and the length of the longest chain in order to get what you need.

Sorry I meant Question 4 part f

Thanks a lot for the help...

 

[m7aug]

can anybody please tell me that how that that whether an organic can be made planar by rotating the bonds ? I saw one such question in a P4 may it was 10 may june somewhere i dont remember

 

[a_wiserME!!]

can someone explain why the answer is C?? TIA

 

[Xaptor16]

For the first one,
Ca(OH)2 + SO2 -------> CaSO3 + H2O
Ca combines with SO2 ---> CaSO2 and then the extra Oxygen atom comes from (OH)2. The other product is water.

The process mentioned above is known as flue-gas desulfurization. Which is dealing with the waste gases that contain SO2. To be more specific, they use wet scrubbing to get rid of the SO2 in the form of CaSO3 which is solid.

CaSO4 can then be formed by oxidizing CaSO3 so that's probably why CaSO3 takes precedence over CaSO4, in this specific reaction. Though generally it is CaSO3 that you want to have form from the reaction between SO2 and Limestone/lime.

----------------------------------

All of the oxides listed are of period 3 elements. I suggest you have a look through the syllabus content and find out what exactly you must know about period 3 elements besides their trends. You are required to know how their oxides react.

All of the above oxides can react with NaOH. Al2O3 is amphoteric so it can react with both acids and bases.

P4O10, SO2 and SiO2 form acidic oxides that can react with bases. The exception here is that SiO2 only reacts with Hot and concentrated NaOH as opposed to the other 3.

Since the question mentions those that "do not" react with cold dilute NaOH, you can with absolute certainty rule out SiO2 and have that as your final answer.

As for understanding why it does not, I recommend looking into how to determine strong and weak acids/bases. So then you can understand whether dilute/concentrated substances can have an effect on the outcome.

For SiO2, from what I know, its oxide is very weakly acidic and can only be neutralized by strong bases such as NaOH. So a dilute NaOH would not have as much of an effect on SiO2 as a hot and concentrated one would.

Hope that made sense!

thankyou very much, i really appreciate it that really helps!

 

[Xaptor16]

CH3CH2CH2CO2CH3 undergoes acid hydrolysis in the stomach under the presence of water, with HCl being the acid catalyst of the hydrolysis.

[CH3CH2CH2CO2] comes from the carboxylic acid and CH3 comes from the alcohol. During esterification, OH is taken from the alcohol and H is taken from -COOH in the carboxylic acid.

When hydrolysis takes place, the OH and H are returned to their respective compounds.

So you end up with CH3CH2CH2CO2H for the carboxylic acid and CH3OH for the alcohol.

They're asking for the empirical formula of the acid, so first find out how many carbons, hydrogens and oxygens [CH3CH2CH2CO2H] has.

That is 4 carbons, 8 Hydrogens and 2 oxygens.

C4H8O2 ------> C2H4O, which is why the answer is C.

ugh i was making a stupid mistake i just realized... thank you!

 

[Xylferion]

View attachment 52129

can someone explain why the answer is C?? TIA

Already explained this. It's the first post on this page.