Acid and Base Ionisation HSC Past Qs help (1 Viewer)

[csi]

Hi guys,

My teacher gave some past HSC Qs as homework, but I'm unsure of how to do some of them. Much appreciated if someone could help

Note: the correct answers have been bolded

1. Which of the following solutions would have the highest pH?
(A) 0.10M HNO3
(B) 0.10M H2SO4
(C) 0.10M HCOOH
(D) 0.10M NaCL

2. Which of the following solutions have the highest pH?
(A) 1.0M acetic acid
(B) 0.10M acetic acid
(C) 1.0M hydrochloric acid
(D) 0.10M hydrochloric acid

3. A sulphuric acid solution has a concentration of 5 x 10^-4 mol L^-1. What is the pH of this solution, assuming the acid is completely ionised?
(A) 3.0
(B) 3.3
(C) 3.6
(D) 4.0

Comment: I tried doing -log[5 x 10^-4] = 4-log 5 which equal to 3.3. I don't know what I have done wrong here.

Thanks guys!

 

[alujjdnd]

1) High pH = more basic. NaCl is neutral while the other options are acids. So NaCl has the highest pH.
2) More basic, as they are all acids we are looking for a lower concentration. Acetic acid is also a weak acid so it has a higher pH as [H3O] is lower (as it does not fully dissociate)
3) pH=-log[H+], but so i think the answers might be wrong.
Hope this helps!

 

[csi]

1) High pH = more basic. NaCl is neutral while the other options are acids. So NaCl has the highest pH.
2) More basic, as they are all acids we are looking for a lower concentration. Acetic acid is also a weak acid so it has a higher pH as [H3O] is lower (as it does not fully dissociate)
3) pH=-log[H+], but so i think the answers might be wrong.
Hope this helps!

v helpful! thanks

 

[jazz519]

1) High pH = more basic. NaCl is neutral while the other options are acids. So NaCl has the highest pH.
2) More basic, as they are all acids we are looking for a lower concentration. Acetic acid is also a weak acid so it has a higher pH as [H3O] is lower (as it does not fully dissociate)
3) pH=-log[H+], but so i think the answers might be wrong.
Hope this helps!

For q3 you haven't factored in that [H+] doesn't always equal to the [acid]. It is diprotic, therefore must be multiplied by 2. So [H+] is given by 2 x 5x10^-4

 

[alujjdnd]

For q3 you haven't factored in that [H+] doesn't always equal to the [acid]. It is diprotic, therefore must be multiplied by 2. So [H+] is given by 2 x 5x10^-4

That explains it! Thanks!~