Soccerboi's questions thread

[charmanderp]

Equivalent protons don't split each other.

[Mao]

Can the carboxyl of the z group react with the amine of an amino acid (amine that's not on the z group) at all?

Of course! it'll form a carboxylate (RCOO-) and an aminium ion (RNH3+), which will then have ionic interactions. This is part of the tertiary structure of proteins.

[soccerboi]

Can the carboxyl of the z group react with the amine of an amino acid (amine that's not on the z group) at all?

Of course! it'll form a carboxylate (RCOO-) and an aminium ion (RNH3+), which will then have ionic interactions. This is part of the tertiary structure of proteins.

I thought the tertiary structure was between the z groups? So even when only one of the groups (either carboxyl or amine) in the z group interacts, it forms the tertiary stucture?

Also, which amino acid would have the highest pH when dissolved in solution?
Histidine or Lysine? Does the NH on histidine contribute to pH because it's a secondary amine, and the single N is a tertiary amine?

[thushan]

Can the carboxyl of the z group react with the amine of an amino acid (amine that's not on the z group) at all?

Of course! it'll form a carboxylate (RCOO-) and an aminium ion (RNH3+), which will then have ionic interactions. This is part of the tertiary structure of proteins.

I thought the tertiary structure was between the z groups? So even when only one of the groups (either carboxyl or amine) in the z group interacts, it forms the tertiary stucture?

Also, which amino acid would have the highest pH when dissolved in solution?
Histidine or Lysine? Does the NH on histidine contribute to pH because it's a secondary amine, and the single N is a tertiary amine?

Secondary and tertiary amines are both basic. As for histidine vs lysine, I remember this qn coming up before and lysine ended up producing a solution of higher pH. Dw about it. You're not expected to know which one is more basic, unless its obvious (sth like glutamic acid vs lysine)

[soccerboi]

Thanks Thushan

[soccerboi]

I'm stuck on this:
A mass of 0.1373 g of freeze-dried sample of shellfish tissue is dissolved in 2.00 mL of nitric
acid, heated for 3 hours at 125°C and transferred to a 500.0 mL volumetric flask, where it is
made up to the mark with deionised water. A volume of 1.00 mL of this solution is then
further diluted to 250.0 mL in a second volumetric flask. AAS is used to measure the
absorbance of this solution, which is found to be 1.03, corresponds to 0.35ug/L.

Q. Calculate the mass, in mg, of mercury in the shellfish sample.
THIS IS WHAT I DID:
m(Hg) in 250ml = 0.35X(250/1000)=0.0875ug
m(Hg) in 1ml = 0.0875ug
m(Hg) in 500ml= 0.0875x(500/1)=43.75 ug=0.04375mg
m(Hg) in 2ml= 0.04375mg=0.044mg(2 sig figs)

What have i done wrong? The answer is 0.022mg

[Lasercookie]

I'm stuck on this:
A mass of 0.1373 g of freeze-dried sample of shellfish tissue is dissolved in 2.00 mL of nitric acid, heated for 3 hours at 125°C and transferred to a 500.0 mL volumetric flask, where it is made up to the mark with deionised water. A volume of 1.00 mL of this solution is then further diluted to 250.0 mL in a second volumetric flask. AAS is used to measure the absorbance of this solution, which is found to be 1.03, corresponds to 0.35ug/L.

Q. Calculate the mass, in mg, of mercury in the shellfish sample.
THIS IS WHAT I DID:
m(Hg) in 250ml = 0.35X(250/1000)=0.0875ug
m(Hg) in 1ml = 0.0875ug
m(Hg) in 500ml= 0.0875x(500/1)=43.75 ug=0.04375mg
m(Hg) in 2ml= 0.04375mg=0.044mg(2 sig figs)

What have i done wrong? The answer is 0.022mg

I think that what you've done is correct.

My stab at what the answers have done is considered the 2mL of nitric acid --> 500mL volumetric flask as a dilution (which would give you 0.022mg), but the amount of Nitric acid is irrelevant to amount of Mercury, so it doesn't need to be considered.

[nisha]

I'm stuck on this:
A mass of 0.1373 g of freeze-dried sample of shellfish tissue is dissolved in 2.00 mL of nitric
acid, heated for 3 hours at 125°C and transferred to a 500.0 mL volumetric flask, where it is
made up to the mark with deionised water. A volume of 1.00 mL of this solution is then
further diluted to 250.0 mL in a second volumetric flask. AAS is used to measure the
absorbance of this solution, which is found to be 1.03, corresponds to 0.35ug/L.

Q. Calculate the mass, in mg, of mercury in the shellfish sample.
THIS IS WHAT I DID:
m(Hg) in 250ml = 0.35X(250/1000)=0.0875ug
m(Hg) in 1ml = 0.0875ug
m(Hg) in 500ml= 0.0875x(500/1)=43.75 ug=0.04375mg
m(Hg) in 2ml= 0.04375mg=0.044mg(2 sig figs)

What have i done wrong? The answer is 0.022mg

I see what you did. The answer is correct (0.022mg)

0.1373g sample-->500ml volumetric flask (n stays the same)
when taking an aliquot of 1.00ml from the diluted sample, concentration stays the same, but the number of mole changes
now diluting the 1.00ml sample to 250ml, changes the concentration but the moles stay the same
(draw a diagram if it gets confusing)

So we basically want to find the concentration of the original sample, so we disregard moles for a second.

heres the calculations:

0.35 *10^-6 g/1000ml, but in 250ml, there are 0.0875 *10^-6 g of shellfish
getting back to the 1.00ml aliquot, we need to multiply by the dilution factor which is 250ml/1ml=*250 (because the 1.00ml sample is MORE CONCENTRATED)
Therefore, 0.0875*250=21.875*10^-6g/1.00ml
=2.1*10^-5g/500ml (as concentration stays the same)
=2.1*10^-2 mg/500ml
=0.022mg / 0.1373g sample

Hope this helps

[soccerboi]

Thanks . So, you worked with the mass of mercury instead of mol right? and treat this mass as if it were concentration so apply dilution factors to the mass when concentrations have changed? I dono if this question makes sense...but yeh.. Thanks for the help,  i see what you have done

[soccerboi]

Q 6a of engage exam, they say that hydrogen bonds are between deoxyribose and phosphate, but they're wrong yeh? its covalent.

[Lasercookie]

Hang on (sorry, I need this to be clarified, I hope I haven't been doing something fundamentally wrong on what should be a simple question). I'll outline my reasoning in full.

So we started off with 0.1373g of sample, being heated and stuff and then put into a 500ml volumetric flask.
So we have 0.1373g in 500mL. Then we take a 1 ml aliquot. So we have less than 0.1373g in 1mL.
We then dilute this to 250mL. Our mass of sample from the 1mL aliquot does not change. Our concentration does, but that's not mass. We then analyse using AAS for how much Mercury there is in this 250mL.

So we have a concentration level of 0.35 micrograms per litre of mercury in 250mL of solution. But what actual mass of mercury do we have in this 250 mL of solution? We have 0.0875 micrograms of Mercury.

We obtained this 250mL solution by diluting a 1mL solution. When you dilute, you don't change the amount of subtance you have. The concentration does change. (, the volume increases and hence concentration decreases when you dilute something). Also note that m=nM, if the amount of moles stays the same, the mass stays the same.

So in the 1mL solution, we have still 0.0875 micrograms of Mercury, but in a higher concentration.

This 1mL solution was obtained from taking an aliquot of a 500mL solution. When we take an aliquot, you do change the amount of substance you have (, the volume decreases AND the amount decreases by the same factor, hence concentration remains the same when you take an aliquot). The concentration of the 1mL solution and 500mL solution remains the same, but the amount of mass will be different.

This means we must calculate the mass of Mercury in the 500ml solution. 0.0875 micrograms * 500 = 43.75 micrograms of Mercury in 500mL.

So we can now say we have 43.75 micrograms of Mercury in 0.1373 g of sample. The answer wanted it in milligrams, so let us convert/apply sig figs: 43.75 micrograms = 0.04375 milligrams = 0.044 milligrams of Mercury in 0.1373 g of sample

[aznxD]

Q 6a of engage exam, they say that hydrogen bonds are between deoxyribose and phosphate, but they're wrong yeh? its covalent.

Yeah it should be covalent.

[Comatose]

how do proteins act as markers for disease? T_T

[destain]

Why is H2NCH2COOH not involved in determining the tertiary structure of a protein?
Isn't it a protein so it's Z group whatever it is will help determine according to its size and attraction to other atoms?

[destain]

how do proteins act as markers for disease? T_T

Diseases can be caused by a defective protein and these proteins are produced from cells that have defective genes Therefore, if you know which proteins are defective, you can identify the disease