Weak Base pH Formula: How To Calculate?

Weak base pH only partially break down into their component ions in an aqueous solution; they're called weak electrolytes and poor conductors of electricity.

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Weak Base pH Formula only partially break down into their component ions in an aqueous solution; they’re called weak electrolytes and poor conductors of electricity. Bases are substances that dissolve in water to form OH- ions and accept hydrogen ions from acids. They cause red litmus paper to turn blue and taste bitter when touched. Weak bases only partially break down into their component ions in an aqueous solution; they’re called weak electrolytes and poor conductors of electricity. In contrast, strong bases dissociate completely into their respective ions upon being dissolved in water. Bases have many practical uses, like lubricants, alkaline batteries, baking soda, and soaps that we use regularly. Pure water can also act as a weak acid and a weak base, while some fruits, such as kiwi and watermelon, are likewise basic in nature. Over the last thousand years, the general properties of acids and bases have been known, but their definitions have drastically changed due to scientific knowledge. When a base is mixed with water, it is called an alkali.

Classification of Weak Base pH

Based on their strength, concentration, and acidity, bases can be broadly classified.

On the basis of Concentration 

Concentrated Base – 

Concentrated bases are aqueous solutions containing a relatively high percentage of the base in the solution. E.g., concentrated ammonium hydroxide, concentrated potassium hydroxide, etc.

Also Read :  Aluminum Acetate

Diluted Base 

A dilute base contains a relatively low percentage of the base in the solution, such as dilute sodium hydroxide, ammonium hydroxide, or potassium hydroxide.

On the basis of Strength

Strong Base – 

That dissolves completely in water. E.g., KOH, NaOH, etc.

Weak Base – 

That doesn’t dissolve completely in water. E.g., NH3, Zn(OH)2

On the basis of the Acidity of Bases 

Monoacidic Base – 

It consists of one hydroxyl ion and one hydrogen ion. For example, sodium hydroxide, ammonium hydroxide, etc.

Diacidic Base – 

Calcium hydroxide, copper (II) hydroxide, etc., contain two hydroxyl ions with two hydrogen ions.

Triacididic Base – 

Aluminium hydroxide, ferric hydroxide, etc., combine three hydroxyl ions with three hydrogen ions.

Also Check – Weak Acid pH Formula

Calculation of Weak Base pH

Part 1: pH

The pH of any solution is defined as the negative log of Hydrogen ion concentration. 

Mathematically, pH = —log10[H+

pH is used to determine the nature of the solution, whether it is acidic, neutral, or basic.

Solution pH range

Acidic  1 – 6.9 

Neutral 7

Basic 7 – 14

Part 2: Identifying the pH of the weak base

Based on the pH values, acidic and basic solutions can be further sub-grouped into strong and weak acids and bases. 

A weak base has a pH value range of 7.1 to 10, whereas a strong base has a pH range of 10.1 to 14.

Examples of Weak Bases pH

NH3 (ammonia)

Ammonia has the chemical formula of NH3 and is a weak base existing as a colorless gas at standard conditions. It’s most distinguishable feature being its pungent odor. Ammonia water, or ammonia solution, contains the ammonium cation (NH4+) and the hydroxide anion. There is partial dissociation of ammonia into these ions when in water.

(N(CH3)3) trimethylamine

In the presence of a catalyst, trimethylamine is produced by reacting ammonia with methanol in the presence of a catalyst. It can also be produced by reacting paraformaldehyde with ammonium chloride.

Pyridine molecule (C5H5N)

It has the chemical formula C5H5N, a heterocyclic compound with a single nitrogen atom instead of one of its methine groups, similar to benzene. The Lewis base properties of the compound allow it to donate electron pairs to Lewis acids, forming Lewis adducts at standard temperatures and pressures. As a result, it appears as a colorless liquid at average temperatures and pressures.

Weak Base pH Formula FAQs

How do you calculate the pH of a weak base solution?

To calculate the pH of a weak base solution, use the equilibrium equation: pH = 14 - pOH, where pOH is the negative logarithm of the hydroxide ion concentration ([OH-]).

What is the best way to determine the concentration of hydroxide ions ([OH-]) in a weak base solution?

Suppose you want to find the concentration of hydroxide ions in a weak base solution. In that case, you can use the equation for ionization of the weak base (B): [OH-] = (Kb/B), where Kb is the weak base dissociation constant and [B] is the weak base's initial concentration.

In a weak base solution, what is the role of the base dissociation constant (Kb)?

The dissociation constant of a weak base in solution (Kb) determines how much the base ionizes. A higher Kb value signifies greater ionization and a stronger base, resulting in a higher concentration of hydroxide ions ([OH-]) and a higher pH. Conversely, a lower Kb value indicates weaker ionization and a lower pH.

Does the Henderson-Hasselbalch equation apply to weak base solutions as well?

Yes, You can apply the Henderson-Hasselbalch equation to weak base solutions. 

How does dilution affect the pH of a weak base solution?

In general, diluting a weak base solution will raise its pH. As more solvent or water is added to the solution, the weak base concentration decreases, leading to a lower concentration of hydroxide ions ([OH-]), and, thus, a higher pH, assuming no other factors are involved, such as strong acids or changes in temperature.


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