VCE Chemistry Question Thread

[Yacoubb]

Can someone confirm what has been aforementioned regarding significant figures after the decimal, for pH values?

[BLACKCATT]

How do we know where hydrogen bonding occurs between the bases? I always seem to get confused about this

[kawfee]

Question

Phenolphthalein indicator was added to an aliquot of sodium carbonate in a flask. HCL acid is added to a burette and titrated against the sodium carbonate. In this titration...

1.endpoint will be the same no matter which indicator is used
b.e/p and the equiv point will be very close to each other
c.e/p will occur well after the equiv point
d.e/p will occur well before the equiv point

what is the answer? and please explain with detail.

Also, could anyone explain the use of indicators, weak acid and strong acid, and theory behind these types of questions? because i'm having a lot of trouble understand it

thank you!

[Yacoubb]

Question

Phenolphthalein indicator was added to an aliquot of sodium carbonate in a flask. HCL acid is added to a burette and titrated against the sodium carbonate. In this titration...

1.endpoint will be the same no matter which indicator is used
b.e/p and the equiv point will be very close to each other
c.e/p will occur well after the equiv point
d.e/p will occur well before the equiv point

what is the answer? and please explain with detail.

Also, could anyone explain the use of indicators, weak acid and strong acid, and theory behind these types of questions? because i'm having a lot of trouble understand it

thank you!

Before I explain it, is the answer C?

[speedy]

This may be a stupid question, I just can't remember it... Can you break up deoxyribose from the base? If so how?

[Yacoubb]

This may be a stupid question, I just can't remember it... Can you break up deoxyribose from the base? If so how?

A nucleotide is synthesised from a base, a deoxyribose sugar molecule and a phosphate group, all of which are covalently bonded in a condensation reaction. Thus, by hydrolysis, you could break existing bonds within a nucleotide to separate the base from the sugar from the phosphate group, and get your separation. Of course hydrolysis is the breaking of an existing covalent bond between two constituent molecules through the introduction of a water molecule.

[speedy]

A nucleotide is synthesised from a base, a deoxyribose sugar molecule and a phosphate group, all of which are covalently bonded in a condensation reaction. Thus, by hydrolysis, you could break existing bonds within a nucleotide to separate the base from the sugar from the phosphate group, and get your separation. Of course hydrolysis is the breaking of an existing covalent bond between two constituent molecules through the introduction of a water molecule.

Yup that's what I thought.

So for this question (bii):

Spoiler

Why is the answer 16? I would have thought it would be 34? 8 + 8 for the phosphates, 9+9 for the bases?

Edit: unless it's got something to do with the wording which I didn't pick up on?

[Yacoubb]

Yup that's what I thought.

So for this question (bii):

Spoiler

Why is the answer 16? I would have thought it would be 34? 8 + 8 for the phosphates, 9+9 for the bases?

Remember that DNA is a double-stranded molecule, and so you'll need 9 water molecules to hydrolyse the first fragment, and another 8 to hydrolyse the complementary fragment. Hence, 8+8=16 molecules of water.

[lzxnl]

Yup that's what I thought.

So for this question (bii):

Spoiler

Why is the answer 16? I would have thought it would be 34? 8 + 8 for the phosphates, 9+9 for the bases?

Edit: unless it's got something to do with the wording which I didn't pick up on?

It says 'nucleotides'; you're only breaking bonds between the phosphates. As for breaking bonds between the bases, those are just hydrogen bonds that don't need water to break.

[Aurelian]

Edit: unless it's got something to do with the wording which I didn't pick up on?

The question asks you about hydrolysis into the constituent nucleotides; thus, this does not involve hydrolysis of the sugar-base bond. It only concerns hydrolysis of the phosphodiester bonds between each nucelotide, of which there are 8 for each strand in fragment A.

[Yacoubb]

It says 'nucleotides'; you're only breaking bonds between the phosphates. As for breaking bonds between the bases, those are just hydrogen bonds that don't need water to break.

Yeah, and we can simply break the hydrogen bonds by heating the DNA fragment.

[speedy]

It says 'nucleotides'; you're only breaking bonds between the phosphates. As for breaking bonds between the bases, those are just hydrogen bonds that don't need water to break.

I meant between the base and deoxyribose - which Yacoubb said is also hydrolysis; so when it says 'nucleotides' it doesn't include between deoxyribose and the base? Or maybe I'm completely lost aha

Edit: Aurelian already answered my question.

[Aurelian]

I meant between the base and deoxyribose - which Yacoubb said is also hydrolysis; so when it says 'nucleotides' it doesn't include between deoxyribose and the base? Or maybe I'm completely lost aha

A nucleotide consists of a base + (deoxy)ribose + a phosphate.

[speedy]

A nucleotide consists of a base + (deoxy)ribose + a phosphate.

Gotcha, thanks

Thankyou to everyone else too

[Yacoubb]

I meant between the base and deoxyribose - which Yacoubb said is also hydrolysis; so when it says 'nucleotides' it doesn't include between deoxyribose and the base? Or maybe I'm completely lost aha

When it says 'nucleotides', it means base+deoxyribose sugar. So, if you want to separate a polynucleotide into its constituent nucleotide monomers, you are essentially breaking the existing bond between the phosphate group and the deoxyribose sugar molecule. If they question asked for how many waters to completely dissociate the molecule (i.e. including nucleotide hydrolysis), you would then to the 16 water molecules, 2 per nucleotide (needed to break the 2 covalent bonds between the sugar and the base, and the sugar and the phosphate group).