General Chemistry

First, recall that a salt is an ionic compound formed by the reaction between an acid and a base where the acid provides the anion, and the base brings the cation to the salt.

 

HCl(aq) + NaOH(aq) →  NaCl(aq) + H2O(l)

 

When the salt is dissolved in water, it dissociates into ions, and depending on which of these ions reacts with water, an acidic or a basic solution is formed. In other words, the pH of the salt solution depends on the type of the acid and the base it is prepared from.

A good shortcut to figuring out if the salt is acidic or basic, is to identify the “strong component” in it.

 

 

Salts the Yield Basic Solutions

For example, potassium cyanide (KCN) is prepared from a strong base (KOH) and weak acid (HCN), and therefore, its aqueous solution will be basic.

So, how do we know that?

The answer is more difficult than what we will do below, however, it is a great way to memorize and answer these types of questions on tests.

The first thing you need to do to figure out if a slat will give an acidic or a basic solution is to determine the acid and base that it is prepared from.

For KCN, it is prepared by the reaction between KOH and HCN, so go ahead and write that reaction:

 

KOH + HCN → KCN + H2O

 

Next, you need to determine if HCN and KOH are strong or weak acid and a base. HCN is a weak acid, while KOH is a strong base, and therefore we conclude that the solution is going to be basic.

The reason for this is that once the salt dissociates, it forms K+ and CN ions, and out of these two, the ion of the weak acid or the base reacts with water producing H3O+ or OH ions.

I know it may be confusing because we said that the shortcut to determining whether the solution is acidic or basic is to find out if it is made of strong acid or a strong base. Yes, it is an easy way to quickly determine the nature of the salt solution, however, it does not explain the reason for such a pattern. And the reason is that the anion of the weak component (acid or base) that forms the salt reacts with water because the anion of a weak acid is a strong base capable of reacting with water.

The concept of acid-base strength is usually not straightforward to grasp, and students have a hard time even when they go over it again in organic chemistry.

So, I’d suggest not to worry too much about it now, but rather go with the shortcut and learn the steps for determining the pH of a salt solution.

For our example, we know, at this point, that it is going to yield a basic solution. We found out that it is made of K+ and CNions, so the next step is to determine the ion of a weaker component and see how it reacts with water.

 

CNis the anion component (conjugate base) of HCN which is the weaker component of KCN, while K+ is the cation of a strong base KOH. Therefore, we will write the reaction of CN ion with water where it acts a base removing a proton from water.

 

In general, remember that bases are negatively charged or at least have a lone pair of electrons that abstracts a proton from the acid, which in this case is the water:

 

 

CN(aq)  + H2O(l) → HCN(aq)  +  OH(aq) (basic)

 

 

So, the reaction of the salt, or more accurately one of the ions of the salts, produces a hydroxide ion which makes the solution basic.

 

To summarize, determine the acid and the base used for preparing the salt; the strong component tells if the solution is going to be acidic or basic.

 

 

Salts the Yield Acidic Solutions

Ammonium chloride (NH4Cl) is the salt of ammonia (NH3) and hydrochloric acid (HCl) and it yields an acidic solution. So, let’s follow what we have discussed earlier and see why it produces an acidic solution.

 

Step 1. Determine the acid and base that the salt is prepared from:

 

NH3 + HCl → NH4Cl

 

It is made of a weak base ammonia and a strong base HCl, and therefore, the solution is going to be acidic. And that’s it – we determined if the solution is basic or acidic. Let’s add step two which is going to be very important for calculating the pH of a salt solution.

 

Step 2. Dissociate the salt and write the reaction of the weak component ion with water.

 

NH4Cl → NH4+ + Cl

 

NH4+ is the ion with a weaker component (NH3), so it is going to react with water producing hydronium ion which makes the solution acidic:

NH4+(aq)  + H2O(l) → NH3(aq)  +  H3O+(aq)  (acidic)

 

In summary, this is what we did:

 

Salts the Yield Neutral Solutions

The table salt NaCl found also in physiological solutions has a negligible effect on the pH of the solution.

Take a few minutes and see if you can figure out why it does not alter the pH.

 

 

Like we did before, determine the acid and the base that make the salt. In this case, it is HCl and NaOH,  which make a strong acid a strong base:

NaCl+(aq) → Na+(aq) + Cl(aq)

 

The salts of strong acids and bases produce neutral solutions. 

The reason for this is that the cation and anion of a salt derived from a strong acid and a strong base, do not react with water and thus, no H3O+ or OH is generated.

 

 

In the next post, we will use this strategy to calculate the pH of salt solutions.

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