Common Ion Effect: pH of LiF, KI & NH4Cl Solutions

• tribalman100
In summary, the addition of LiF to an HF solution causes the pH to increase due to the increase in F- concentration, while the addition of KI to an HI solution does not affect the pH and the pH decreases when NH4Cl is added to an NH3 solution. This is due to changes in equilibrium and the Law of Mass Action.
tribalman100

Homework Statement

does the ph increase , decrease or remain the same on addition of each of the folling
a) LiF to an HF Solution
B) KI to an HI solution
C) NH4Cl to an NH3 solution

The Attempt at a Solution

i got the equations and the answers to these i just want to know the reason so i can do it next time on why Lif ph increases, Ki's remains the same and that NH4Cl's decreases . it would be a great help and this explanation might save my chemistry grade. thanks for the help guys

well, think in terms of equillibria.

HF <------> H+ + F-
LiF <------> Li+ + F-

when you add LiF, the total concentration of F- increases in solution. the dynamic equilibirum of HF is disturbed. according to LCP the backward reaction is momentarily favoured to decrease F- concentration. at the same time, the concentration of H+ also decreases. hence the pH increases. (more alkaline)

try to figure out the others using the same logic.

The common ion effect is a phenomenon in which the presence of a common ion in a solution affects the solubility and pH of a compound. In the case of LiF, KI, and NH4Cl solutions, the common ion is the fluoride ion (F-), iodide ion (I-), and ammonium ion (NH4+), respectively.

When LiF is added to an HF solution, the fluoride ion is already present in the solution as a common ion. This leads to a decrease in the solubility of LiF, causing the pH to increase. This is because the common ion (F-) decreases the concentration of F- ions in the solution, which shifts the equilibrium of the reaction to the left, resulting in a lower concentration of H+ ions and a higher pH.

When KI is added to an HI solution, the iodide ion is already present in the solution as a common ion. However, in this case, the common ion (I-) does not have a significant effect on the solubility of KI. Therefore, the pH of the solution remains relatively unchanged.

When NH4Cl is added to an NH3 solution, the ammonium ion (NH4+) is converted to ammonia (NH3) through a dissociation reaction. This leads to an increase in the concentration of ammonia, which is a weak base. As a result, the pH of the solution decreases as the ammonia reacts with water to form ammonium hydroxide, which is a weak acid. This decrease in pH is due to the common ion (NH4+) increasing the concentration of a weak acid in the solution.

What is the common ion effect?

The common ion effect is when the addition of a common ion (an ion that is already present in the solution) causes a decrease in the solubility of a salt or a decrease in the ionization of a weak acid or base. This is due to the Le Chatelier's principle, which states that a system at equilibrium will shift to counteract any changes made to the system.

How does the common ion effect affect the pH of a solution?

The common ion effect can affect the pH of a solution by decreasing the concentration of hydrogen ions (H+) or hydroxide ions (OH-) in the solution. This is because the addition of a common ion can shift the equilibrium of a weak acid or base, resulting in a decrease in the concentration of H+ or OH- ions and a subsequent change in the pH of the solution.

What is the pH of a LiF solution?

The pH of a LiF (lithium fluoride) solution will be basic due to the strong base properties of the fluoride ion. However, the exact pH will depend on the concentration of the solution and the presence of any other ions that may contribute to the common ion effect.

Why does the common ion effect occur in solutions of KI and NH4Cl?

The common ion effect occurs in solutions of KI (potassium iodide) and NH4Cl (ammonium chloride) because both of these salts contain ions that are also present in water. The addition of these salts to a solution will increase the concentration of these common ions, causing a shift in equilibrium and a subsequent decrease in the solubility or ionization of other compounds in the solution.

How can the common ion effect be used to manipulate the pH of a solution?

The common ion effect can be used to manipulate the pH of a solution by adding a common ion to the solution. For example, if a solution is too acidic, one can add a salt that contains a basic ion such as OH- or CO3- to decrease the concentration of H+ ions and increase the pH. Conversely, if a solution is too basic, one can add a salt that contains an acidic ion such as H+ or NH4+ to decrease the pH.

• Biology and Chemistry Homework Help
Replies
1
Views
4K
• Biology and Chemistry Homework Help
Replies
20
Views
6K
• Biology and Chemistry Homework Help
Replies
5
Views
2K
• Biology and Chemistry Homework Help
Replies
5
Views
2K
• Biology and Chemistry Homework Help
Replies
8
Views
2K
• Biology and Chemistry Homework Help
Replies
1
Views
5K
• Biology and Chemistry Homework Help
Replies
4
Views
2K
• Biology and Chemistry Homework Help
Replies
10
Views
4K
• Biology and Chemistry Homework Help
Replies
6
Views
13K
• Biology and Chemistry Homework Help
Replies
1
Views
3K