ALL CIE CHEMISTRY DOUBTS HERE !!

[elemis]

That's true but before turning into a carboxylic acid......the compound will first be converted into an aldehyde. We need to oxidise much more for it to get converted to carboxylic acid.

The question says it is oxidised completely THEN it is tested with DNPH

[Deadly_king]

The question says it is oxidised completely THEN it is tested with DNPH

Yupz........but note that in test 5, the reaction says warm with acidified dichromate ions. This is a weak reagent which is not sufficiently strong to convert the aldehyde formed to carboxylic acid readily.

If manganate ions had been used, then no aldehyde would have been present since all of them would have readily been converted to carboxylic acid.

This is the difference, your answer would have been okay had manganate ions been used instead of dichromate.

[narnia]

Why is SiO₂ insoluble in water buh dissolves in concentrated alkali

Also,why acid-base character change as we go down group4?

[Amelia]

Why is SiO₂ insoluble in water buh dissolves in concentrated alkali

Silicon dioxide is a giant molecule (macromolecular structure due to extensive network of covalent bonds in the crystalline lattice structure that extends to infinity). since the solubility of a substance is related to the similarity in bond strength in the solvent (water in this case) AND the solute (silicon dioxide in this case), and the extensive network of covalent bonds is much stronger than the water-water interactions (i.e. hydrogen bonds), therefore silicon dioxide CANNOT be hydrated by water molecules (i.e. water cannot break down the giant crystal lattice structure of the macromolecule SiO2) to form aqueous ions and therefore it remains insoluble in water. (silicon dioxide only dissolves by heating with strong alkali. in this case water is not alkaline enough to hydrolyse silion dioxide to give sillicate ions).
Also Silicon dioxide is an acidic compound.
In one line,  Silicon dioxide doesn't react with water, because of the difficulty of breaking up the giant covalent structure.

Also,why acid-base character change as we go down group4?

http://www.thestudentroom.co.uk/wiki/Revision:Acid-Base_Character_of_Group_4_Oxides

[Nera_egypt]

I have a question in the application booklet related to Biochemistry

In section 1.3 – Genetic information. I couldn't solve  SAQ 11 part c on page 41 .
I would be really glad if someone helps me work it out 


P.S: I attached the question

[tmisterr]

I have a question in the application booklet related to Biochemistry

In section 1.3 – Genetic information. I couldn't solve  SAQ 11 part c on page 41 .
I would be really glad if someone helps me work it out 


P.S: I attached the question

from the peptide fragment, we see -Ala-Ala-: two same amino acids one after another. so we would expect that we see two same codons following each other on the fragment of mRNA. the first time this happens is -GCU-GCU- so we can say that the codon for Ala is GCU. we can confirm this by skipping the next two codons (-GAA-GGA-), which we can suspect to be for the -Glu-Gly- section of the peptide and we see that the next codon is again -GCU- confirming that GCU is the codon for Ala since is coincides with the polypeptide chain, that it two Ala, followed by two other amino acids then Ala again. once you know this, you can easily get the rest

(ii) deciphering gives the codon for tyrosine as UAC. this is for the mRNA so the DNA triplet code from which it was transcribed from is ATG (I hope you know how the bases match). since mRNA is always synthesised from the 5' end to the 3'end the direction of this codon is 5'-ATG-3'

well then, I hope this helps

[tmisterr]

i'd also like to emphasize that there is no need to memorize codons, you will be given the genetic code which has all the codons if you are needed to use it in exams so no need to stress yourself on that. just understand how transcription and translation work and you'll be fine

[Nera_egypt]

from the peptide fragment, we see -Ala-Ala-: two same amino acids one after another. so we would expect that we see two same codons following each other on the fragment of mRNA. the first time this happens is -GCU-GCU- so we can say that the codon for Ala is GCU. we can confirm this by skipping the next two codons (-GAA-GGA-), which we can suspect to be for the -Glu-Gly- section of the peptide and we see that the next codon is again -GCU- confirming that GCU is the codon for Ala since is coincides with the polypeptide chain, that it two Ala, followed by two other amino acids then Ala again. once you know this, you can easily get the rest

(ii) deciphering gives the codon for tyrosine as UAC. this is for the mRNA so the DNA triplet code from which it was transcribed from is ATG (I hope you know how the bases match). since mRNA is always synthesised from the 5' end to the 3'end the direction of this codon is 5'-ATG-3'

well then, I hope this helps

Thanks alot, this was the best explanation given to me  

[narnia]

Silicon dioxide is a giant molecule (macromolecular structure due to extensive network of covalent bonds in the crystalline lattice structure that extends to infinity). since the solubility of a substance is related to the similarity in bond strength in the solvent (water in this case) AND the solute (silicon dioxide in this case), and the extensive network of covalent bonds is much stronger than the water-water interactions (i.e. hydrogen bonds), therefore silicon dioxide CANNOT be hydrated by water molecules (i.e. water cannot break down the giant crystal lattice structure of the macromolecule SiO2) to form aqueous ions and therefore it remains insoluble in water. (silicon dioxide only dissolves by heating with strong alkali. in this case water is not alkaline enough to hydrolyse silion dioxide to give sillicate ions).
Also Silicon dioxide is an acidic compound.
In one line,  Silicon dioxide doesn't react with water, because of the difficulty of breaking up the giant covalent structure.

http://www.thestudentroom.co.uk/wiki/Revision:Acid-Base_Character_of_Group_4_Oxides

Thanks tons!

[EMO123]

What are the Friedel-Crafts Catalyst?

[Amelia]

What are the Friedel-Crafts Catalyst?

They are the catalysts used in the Friedel-Crafts reactions.

Here are examples of the reactions - http://www.chemguide.co.uk/organicprops/arenes/fc.html

 

Thanks tons!

Welcome:)

[EMO123]

They are the catalysts used in the Friedel-Crafts reactions.

Here are examples of the reactions - http://www.chemguide.co.uk/organicprops/arenes/fc.html

 
Welcome:)

thanxxs

[TheLonelyIsland]

Hi, can someone PLZ help me with this question? I'm so stupid, it's really easy, but I keep getting the wrong answer    :

"Which of these samples of gas contains the same number of atoms as 1g of hydrogen           
(Mr : H2, 2)?
A 22 g of carbon dioxide (Mr: CO2, 44)
B 8 g of methane (Mr: CH4, 16)
C 20 g of neon (Mr: Ne, 20)
D 8 g of ozone (Mr: O3, 48) "

According to ms, the correct answer is C...How do you do it correctly? Arghh, so frustrating when you don't know, lol...

Thx in advance,

Take care

[Deadly_king]

Hi, can someone PLZ help me with this question? I'm so stupid, it's really easy, but I keep getting the wrong answer    :

"Which of these samples of gas contains the same number of atoms as 1g of hydrogen           
(Mr : H2, 2)?
A 22 g of carbon dioxide (Mr: CO2, 44)
B 8 g of methane (Mr: CH4, 16)
C 20 g of neon (Mr: Ne, 20)
D 8 g of ozone (Mr: O3, 48) "

According to ms, the correct answer is C...How do you do it correctly? Arghh, so frustrating when you don't know, lol...

Thx in advance,

Take care

Hmm..........it's not very difficult.

The most important thing that you need to know is that 1 mole of any substance contains 6.02 x 10²⁴ number of atoms.

So you've been asked to find a sample with the same number of atoms as 1g of hydrogen. 1 gram of Hydrogen as you should be knowing is equal to 1 mole of hydrogen (Ar of H = 1).

In other words, now you need to look for a sample representing one mole. You can use the following formula to do do so.

No of moles = Mass / Mr

A ----> No of moles = 22/44 =0.5
B ----> No of moles = 8/16 = 0.5
C ----> No of moles = 20/20 = 1
D ----> No of moles = 8/48 = 0.17

You'll note that only C offers this possibility.

Hope it helps

[TheLonelyIsland]

thx a lot, i knew it was something really easy... What confused me was that Mr H2=2... Dumb, I know! Thx, again!