How do I know if a solution is acidic, basic, or neutral?

Question

I get by looking at pH's you can tell, but given just the compound say LiNO2, LiNO3, KCN, NaClO, how do I know that KCN has the highest PH and is most basic? Is this based on the periodic table? It says each of the salts were dissolved in water for a 0.10M solution and I know the answer is KCN but I'm just not sure how. Same when it asks for lowest pH.

Answer 1

The easiest way to tell is to look at each salt and determine how it was made, i.e. from which acid and which base. Because, of course, all these salts come from an acid + a base.

If the salt comes from a STRONG acid and weak base, the pH will be acidic (<7)

If the salt comes from s STRONG base and a weak acid, the pH will be basic (>7)

If the salt comes from a STRONG acide and a STRONG base, the pH will be neutral (7)

From your list we have....

LiNO2 which comes from a strong base (LiOH) and a weak acie (HNO2). pH is basic (>7)

LiNO3 which comes from a strong base (LiOH) and a strong acid (HNO3). pH is neutral (7)

KCN which comes from a strong base (KOH) and a weak acid (HCN). pH is basic (7)

NaClO which comes from a strong base (NaOH) and a weak acid (HClO). pH is basic (>7).

To determine which is most basic among the three (LiNO2, KCN and NaClO) one must compare the Kb of the conjugate base and that will dictate the level of alkalinity.