VCE Stuff > VCE Chemistry
In Need of Urgent Help
Collin Li:
Bear: I'm not sure if it's the charge on NH3+, it should be the charge of the overall solution (because both the neutral and acidic forms are present at the same time). The charge on NH3+ will always just be +1.
I will find the charge of the overall solution:
For pH = 6.6:
This means there is a 1:1000 ratio of the base and the acid. This means that there is % of the +1 charged species, and % of the neutral species. This means the overall 'average' charge on the amino acid is to 3 significant figures.
This makes sense. In a more acidic solution (lower pH), the protonated species is more abundant, so we have a higher positive charge.
For pH = 12.6:
This means there is a 1000:1 ratio of the base and the acid. This means that there is % of the +1 charged species, and % of the neutral species. This means the overall 'average' charge on the amino acid is to 3 significant figures.
This makes sense. In a more basic solution (high pH), the protonated species is less abundant, so we have a smaller positive charge.
Bear:
Thanks for that, but in the next bit of working they started calculating and they started getting charges in NEGATIVE
Like Carboxyl group has pKa = 2.3
at pH = 2.3, charge = -0.5 etc...
why is it suddenly negative? i thought because the carboxyl group is a negative charged ion, thats why..and Nh3+ is a postitively charged ion.
Collin Li:
Ah, because the amino acid can also take this combination:
H2N-CH2-COOH (neutral or "acidic")
H2N-CH2-COO- (conjugate base - after it has donated its proton)
The neutral amino acid can act as an acid or a base. In this case, the amino acid is acting as an acid (it was acting as a base before).
So now the acid is the neutral form, and the base is the negatively charged species, so when you find out the ratio for them, you will end up with an overall negative charge in the solution.
jeremiahk:
Nothing on the new study design indicates or suggests this sort of increase in difficulty of acid/base questions. However I have had a few students present these sorts of "equilibrium" changes with using K values in more advanced ways. They ALL were from high performing schools.
Bear:
Can you please show me how do you know whether the amino acid is acting as a acid or base? Because all the information we got was the formula for Glycine. Can you show me the working when we are finding the overall charge when the amino acid is acting as a base? Say, when pH = 4.3? Thanks Coblin.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version