Ka chemistry

What is K_a in Chemistry?

The acid dissociation constant (Ka) is used to differentiate between strong and weak acids. The acid dissociates more as the Ka increases. Strong acids must therefore dissociate more in water. A weak acid, on the other hand, is less likely to ionise and release a hydrogen ion, leading to a less acidic solution.

The acid dissociation constant, denoted by Ka, is the equilibrium constant of an acid’s dissociation reaction. This equilibrium constant is a numerical representation of an acid’s strength in a solution. Ka is often stated in mol/L units.

Table of Contents

• Relating K_a and pK_a
• Using K_a and pK_a To Predict Equilibrium and Strength of Acids
• Acid Dissociation Constant From pH
• K_a Example
• Frequently Asked Questions – FAQs

Relating K_a and pK_a

The equilibrium of acid dissociation in aqueous solution can be stated symbolically as:

HA + H₂O ⇆ A^– + H₃O⁺

Where HA is an acid that dissociates into its conjugate base A^–, and H⁺ is a hydrogen ion that joins with a water molecule to form H₃O⁺.

When the concentrations of the chemical species HA, A, and H₃O⁺ do not fluctuate with time, they are said to be in equilibrium. The equilibrium concentrations (in mol/L), represented by [HA], [A], and [H₃O⁺], are commonly written as a quotient of the dissociation constant Ka.

Ka = [A^–][H₃O⁺] / [HA][H₂O]

The concentration of water does not change significantly as it reacts with the acid in most circumstances (unless in the most concentrated aqueous solutions of an acid). As a result, it can be treated as a constant and ignored.

HA ⇆ A^– + H⁺

Ka = [A^–][H⁺]/[HA]

The reaction and definition can then be written in a more straightforward manner.

pKa = – log10Ka

It is more convenient to discuss the logarithmic constant, pKa, for many practical uses. The relationship between Ka, pKa, and acid strength is as follows: the lower the Ka value, the higher the pKa value, and the weaker the acid.

Predict Equilibrium and Strength of Acids Using Ka and pKa

Ka can be used to calculate the equilibrium position:

• When Ka is big, the creation of dissociation products is encouraged.
• When Ka is low, the undissolved acid takes precedence.

Ka can be used to estimate an acid’s strength:

• If Ka is high (and pKa is low), the acid is largely dissociated and therefore powerful. Strong acids have a pKa less than or equal to -2.
• When Ka is low (and pKa is high), there has been little dissociation, hence the acid is weak. Weak acids have a pKa in water that ranges from -2 to 12.

Because adding water to an acid solution does not change its acid equilibrium constant, but it does modify the H+ ion concentration and pH, Ka is a better estimate of an acid’s strength than pH.

Acid Dissociation Constant From pH

The pH scale, or “power of hydrogen,” is a numerical measure of a solution’s acidity or basicity. In an aqueous solution, it can be used to compute the concentration of hydrogen ions [H⁺] or hydronium ions [H₃O^+]. Low pH solutions are the most acidic, whereas high pH solutions are the most basic.

The concentration of free hydrogen (or hydronium) ions in an aqueous acid solution is measured by its pH: pH equals -log [H⁺] or -log [H₃O⁺]. The last equation can be rewritten as follows:

The above comparison allows you to determine the relative concentration of acid to conjugate base and estimate the dissociation constant Ka if you know the molar concentration of an acid solution and can detect its pH.

Ka Example

Question:

If the hydroxide ion concentration is 6.6×10^-6M, what is the concentration of hydronium ions in a solution?

Solution:

The product of the hydroxide ion concentration and the hydronium ion concentration in any acidic or basic solution equals 1×10^-14, the dissociation constant for water. To put it another way:

2H₂O ⇌ H₃O⁺ + OH⁻

Ksp = [H₃O⁺][OH⁻] = 1∗10⁻¹⁴

We can use this equation to calculate the hydronium ion concentration if we know the hydroxide ion concentration.