pH of salt solutions (1 Viewer)

[.ben]

Can someone tell me the wheter each of the following substance is acidic/basic and its parent acid and bases please.

-ammonium chloride (NH₄Cl)
-copper sulfate (CuSO₄)
-Potassium Thiocyanate (KSCN)
-Potassium Chloride (KCl)

thank you

 

[tennille]

You can view the parent acids/bases of the cations and anions involved here. The cations involved in the salts would most probably be hydroxides.
To determine whether the salt solutions will be acidic or basic, you need to use these rules:

A salt formed from a strong acid and a strong base
There will be no acid-base properties, therefore the solution will be neutral.

A salt formed from a strong acid and a weak base
The conjugate base to the strong acid (the anion of the salt) will have no basic properties. The conjugate acid to the weak base (the cation) will have significant acidic property. Therefore, this salt solution will be acidic.

A salt formed from a weak acid and a strong base
The conjugate acid to the strong base (the cation of the salt) will have no acidic properties. The conjugate base to the weak acid (the anion) will have basic properties. Thus, this salt solution will be basic.

A salt formed from a weak acid and a weak base
The conjugate acid to the weak base (the cation) will have acidic properties and the conjugate base to the weak acid (the anion) will have significant basic properties. As a result, both the cation and anion hydrolyse and the effect really depends on which is the stronger acid.

 

[.ben]

Wow! i never thought of looking at salt soluns this way. Very useful thanks

btw for the weak acid and weak base --> isn't it a strong conjugate base and strong conjugate acid --> so neutral salt?

but for SCN i still can't find the parent acid

 

[Riviet]

but for SCN i still can't find the parent acid

To find the parent acid, just give it a hydrogen ion:
KSCN^- + H⁺ -> HKSCN

 

[.ben]

O, cos i was thinking more along the lines of potassium hydroxide and something thiocyanate

i foudn that the parent acid of SCN was HSCN thiocyanic acid.

 

[[Damo]]

One thing need to remember:

Basic salts: The anion is a weak base. Eg. NaHSO4

Acidic salts: The cation is a weak acid. Eg KNO2

 

[[Damo]]

sorry other way around,

acidic salt: NaHSO4

The HSO4- ion donates proton

Basic KNO2

The NO2- accepts a proton