What is Dissociation?
A dissociation reaction occurs when water splits into hydrogen and hydroxide ions. When a molecular compound dissociates into ions, the reaction is known as ionisation. When acids dissociate, hydrogen ions are produced.
A dissociation reaction is a chemical reaction that occurs when a compound separates into two or more components.
A dissociation reaction’s general formula is as follows:
AB → A + B
Dissociation reactions are usually chemical reactions that can be reversed. When there is only one reactant but multiple products, this is an indication of a dissociation reaction.
Water molecules break apart the ionic crystal when ionic compounds dissociate. This is due to the attraction of the positive and negative ions in the crystal, as well as the negative and positive polarity of water.
While writing a dissociation reaction in which a compound is broken down into its constituent ions, charges are placed above the ion symbols and balance the equation for both mass and charge.
Table of Contents
How to Calculate Percent Dissociation?
Strong acids and bases completely dissociate when placed in water. That is, the entire acid [HA] or entire base [BOH] separates into ions.
Weak acids and weak bases, on the other hand, do not completely dissociate in an aqueous solution. The dissociation constant Ka and Kb describes the extent to which they separate:
Ka = ([H+][A–]) ÷ [HA]
Kb = ([B+][OH–] ÷ [BOH]
Ka and Kb can be used to calculate the percentage of a weak acid or weak base that is dissociated in a solution with a known pH and pOH.
A symbol that denotes percent dissociation is α (alpha). Dissociation is simply the conversion of a compound into its respective ions in a suitable solvent. The percentage of dissociation of a compound will vary depending on the solvent.
Step to calculate percent dissociation-
- To calculate the percentage of dissociation of a compound, divide the mass of the dissociated ions by the total mass of the dissociated and not dissociated compound, then multiply by 100.
- In general, not all compounds will be soluble in all solvents. Few compounds will be soluble in the sense of dissociating into their respective ions.
- If we calculate the percent dissociation of acid, we call it acid dissociation, and if we calculate the percent dissociation of a base, we call it base percent dissociation.
- The percentage of dissociation will be determined by the concentration of the acid, base, or neutral compound.
- The percentage of dissociation of an acid or a base will be determined by its strength.
- If the acid is too strong, it will have a high percentage of dissociation, whereas if the acid is too weak, it will have a low percentage of dissociation. In the same way, the percentage of dissociation will apply to the strength of the base.
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Solved Examples
Example 1:
A weak acid has a pKa of 4.994 and the solution pH is 4.523. What percentage of the acid is dissociated?
Solution:
1. Convert pKa to Ka:
Ka = 10-pKa = 10–4.994 = 1.0139 x 10–5
HA ⇌ H+ + A–
The equilibrium expression for that dissociation is as follows:
Ka = ([H+] [A–]) / [HA]
2. [H+] = 10–pH = 10–4.523 = 3.00 x 10–5 M
We know that [A–] = [H+] = 3.00 x 10–5 M because of the 1:1 molar ratio in the above equation.
3. Calculate [HA] by the equilibrium expression.
1.0139 x 10–5 = [(3.00 x 10–5) (3.00 x 10–5)] ÷ x
x = 8.88 x 10–5 M
4. The percent dissociation of acid is [H+] ÷ [HA] multiplied by 100.
α = (3.00 x 10–5 ÷ 8.88 x 10–5) × 100 = 33.8%.
Example 2:
Calculate the percent dissociation of a weak acid in a 0.060M solution of HA (Ka = 1.5 × 10–5).
Solution:
1. Calculate the concentration of H+.
Ka = ([H+] [A–]) / [HA]
1.5 × 10–5 = x2 ÷ 0.060 – x
2. Since, the acid dissociates to a very small extent, it can be assumed that x is small. Therefore, the above equation can be written as-
1.5 × 10–5 = x2 ÷ 0.060
x = [H+] = 9.4 × 10–4
3. To calculate the percent dissociation, divide the concentration of the hydrogen ion by the concentration of the undissociated species, HA, and multiply by 100 percent:
α = (9.4 × 10–.4 ÷ 0.060) × 100 = 1.6%.
Frequently Asked Questions on Percent Dissociation Formula
What is the formula for calculating percent dissociation?
Divide the total mass of dissociated and undissociated species by the mass of dissociated ions, then multiply by 100 percent.
What factors influence percent dissociation?
The percentage dissociation of acid varies with its concentration. The higher the percentage of ionisation, the more dilute the acid.
How can you increase the percentage of dissociation?
Diluting a weak acid causes the concentration of a reaction’s products to shift to the left, causing the acid to dissociate more in order to reestablish equilibrium, increasing the percent dissociation.
Write down the factors that influence the degree of ionisation.
Factors influencing the degree of dissociation include:
- Nature of solute: Strong electrolytes have a degree of dissociation close to one, while weak electrolytes have a degree of dissociation less than one.
- Nature of solvent: A solvent with a high dielectric constant has a higher degree of dissociation.
- Dilution: As the solution is diluted, the degree of dissociation increases.
- Temperature: As the temperature rises, so does the degree of dissociation.
- Other species addition: The addition of another solute with an ion similar to that of weak electrolyte results in a decrease in the degree of dissociation of the weak electrolyte.
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