Base Dissociation Constant (Kb) - Chemistry

1. Fundamental Concepts

Kb = Base Dissociation Constant: quantifies the extent to which a weak base accepts H⁺ from water to produce OH⁻.
Applies only to weak bases (strong bases dissociate completely, so no Kb needed).
General reaction:
Equilibrium expression:
Larger Kb = stronger weak base; smaller Kb = weaker weak base.

Easy

A weak base B has an initial concentration of and .

Calculate .

1. Reaction:

2. Let ,

3. $\text{OH}^-] = \sqrt{K_b \cdot c} = \sqrt{(1.0\times10^{-5})(0.10)} = 1.0 \times 10^{-3}\ \text{M}$ (Assuming , we use )

Answer:

Medium

For , .

Calculate the pH.

1. $\text{OH}^-] = \sqrt{K_b \cdot c} = \sqrt{(1.8\times10^{-5})(0.20)} \approx 1.90 \times 10^{-3}\ \text{M}$

Answer:

Hard

A weak base solution has .

Find .

3. ,

Answer:

1. Write the base dissociation equilibrium.

2. Define .

3. Set up the expression.

4. Use the approximation if valid.

5. Solve for , then find pOH and pH.

6. Use for conjugate acid-base problems.