Chemistry 3/4 2013 Thread

[thushan]

How about a pH question?
1X10^-4 M CH3COOH solution? I'm stuck because none of my answers fit the MC options (4,5,7,9)

This is a process of elimination. If it were hcl then pH is 4. Since acetic acid is weak, yet is nevertheless an acid, you are looking for a solution with pH between 4 and 7.

[Edward21]

This is a process of elimination. If it were hcl then pH is 4. Since acetic acid is weak, yet is nevertheless an acid, you are looking for a solution with pH between 4 and 7.

Oh I see, yes. How about "assuming that the differences in the strengths of H2SO4 and HCl are negligible, which of the following solutions (if any) would produce the solution with the lowest pH?
0.15M H2SO4 OR 0.30M HCl I'm not exactly sure what this question is trying to get at

[teletubbies_95]

Hey thanks for asking the question!

First of all , Hcl and h2so4 are strong acids.
But h2so4 is a diprotic strong acid, therefore will have double the concentration of hydronium ions than hcl , which is monoprotic strong acid.
But according to the question , they have the same molarity! Sos.

Hope that helps!!

[Edward21]

Hey thanks for asking the question!

First of all , Hcl and h2so4 are strong acids.
But h2so4 is a diprotic strong acid, therefore will have double the concentration of hydronium ions than hcl , which is monoprotic strong acid , therefore having a lower pH .

Hope that helps!!

But isn't the hydronium ion concentration in both identical, 2 X 0.15 = [H3O+] = 0.30M, identical with the molarity of hydronium ions in the 0.30M HCl, or is it just testing your knowledge that even though they have the same hydronium ion molarity, H2SO4 will still have a lower pH being a diprotic acid?

[teletubbies_95]

Yes, they do have the same molarity. But they are both strong acids, therefore they have similar dissociation constants( which are constant equilibrium values measuring the ability for larger molecules to dissociate to smaller molecules) , therefore would have similar ph !

[thushan]

Hey thanks for asking the question!

First of all , Hcl and h2so4 are strong acids.
But h2so4 is a diprotic strong acid, therefore will have double the concentration of hydronium ions than hcl , which is monoprotic strong acid.
But according to the question , they have the same molarity! Sos.

Hope that helps!!

Careful guys.

0.30 M HCl vs 0.15 M H2SO4 - the 0.30 M HCl will have the higher [H+].

Yes, the strengths of H2SO4 and HCl are practically the same. However, HSO4- is a weak acid.

0.30 M HCl will have all the HCl ionised => [H+] = 0.30 M
0.15 M H2SO4; all the H2SO4 ionises, but only SOME of the HSO4- will subsequently have its protons removed. Hence, in this case, 0.15 M < [H+] < 0.30 M.

Rhinoking - remember that by definition pH = -log₁₀[H+]. Hence, if the [H+] in both acids are the same, it will necessarily mean that pH will be the same.

[teletubbies_95]

Why does that happen ?  I mean , all the h2so4  ionising and only some donating protons.
And why can't it happen with Hcl?

I'm Sorry , just confused!

[thushan]

Okay, so let's dissolve H2SO4 in water.

We get this reaction:

H2SO4 + H2O --> HSO4- + H3O+

This is to completion, everything is fine and dandy. So, we have water, H3O+ and HSO4- in solution.

Now, this HSO4- can participate in a second reaction:

HSO4- + H2O <--> SO4 2- + H3O+

This reaction is reversible.

[Edward21]

Okay, so let's dissolve H2SO4 in water.

We get this reaction:

H2SO4 + H2O --> HSO4- + H3O+

This is to completion, everything is fine and dandy. So, we have water, H3O+ and HSO4- in solution.

Now, this HSO4- can participate in a second reaction:

HSO4- + H2O <--> SO4 2- + H3O+


This reaction is reversible.

Yes I totally understand thank you so much, !

[thushan]

Yes I totally understand thank you so much, !

No worries man!

[teletubbies_95]

Sorry rhinoking for confusing you! And thanks thush!!!

[thushan]

Sorry rhinoking for confusing you! And thanks thush!!!

No worries - you like your new name?

[teletubbies_95]

Why you change it thush?    :-XLol!

Anyway, i like it. Thanks!

[thushan]

 

Hehe. Self explanatory.

[Edward21]

Sorry rhinoking for confusing you! And thanks thush!!!

That's ok, you learn way more from your failures than your successes is what I've decided