15.7
Weak Base Solutions
Some compounds produce hydroxide ions when dissolved by chemically reacting with water molecules. In all cases, these compounds react only partially a…
A weak base, like ammonia, is a Brønsted base that accepts a proton from water to produce the hydroxide ion. Weak bases react partially with water according to their base dissociation constant, Kb, which is 1.76 × 10−5 for ammonia.
The Kb for ammonia can be expressed as the ammonium ion concentration times the hydroxide ion concentration divided by the concentration of ammonia at equilibrium.
Kb can be used to determine the hydroxide ion concentration in a weak base solution and consequently, the pOH and pH of the solution.
The hydroxide ion concentration and pH of 0.23 M of ammonia solution can be determined using its base dissociation constant and by preparing an ICE table containing the initial and equilibrium values of the ammonia, ammonium ions, and hydroxide ions.
Substituting equilibrium concentrations in the Kb expression, Kb equals x times x divided by 0.23 minus x. As weak bases show partial dissociation, 0.23 minus x can be considered to be approximately 0.23.
When the equation is solved, x equals 2 × 10−3 M.
The approximation 0.23 minus x is equal to 0.23 is valid here as the hydroxide ion concentration is only 0.86% of 0.23 molar.
To calculate the pH of this solution, first determine the pOH by taking the negative log of the hydroxide ion concentration, which equals 2.70. The pH can be determined using the formula: pH plus pOH is equal to 14 and calculated to be 11.30.
The Kb for a weak base solution can be calculated if the pH of the weak base solution is known.
Methylamine is a weak base that partially dissociates in water into methylammonium ions and hydroxide ions.
The Kb for methylamine can be expressed as the methylammonium ion concentration times the hydroxide ion concentration divided by the concentration of methylamine at equilibrium.
To calculate the Kb of a 0.040 M methylamine solution with pH 11.6, first the pOH needs to be calculated followed by its hydroxide ion concentration. As the pH is 11.60, the pOH is 2.40, and its hydroxide ion concentration is 4.0 × 10−3.
The ICE table can be constructed from the initial and the equilibrium concentrations of methylamine, methylammonium, and hydroxide.
Substituting equilibrium concentrations in the expression for the Kb yields value for Kb, which is 4.4 × 10−4.
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Q1: What is a weak base and how does it differ from a strong base?
A weak base is a Brønsted base that accepts a proton from water to produce hydroxide ions, but reacts only partially with water. Unlike strong bases that completely dissociate, weak bases establish an equilibrium between the molecular form and ionized products. Ammonia is a common example, with only about 1% of dissolved ammonia present as ammonium ions under typical conditions.
Q2: How is the base dissociation constant used to calculate hydroxide ion concentration?
The base dissociation constant (Kb) expresses the ratio of product concentrations to reactant concentration at equilibrium. For ammonia, Kb = 1.76 × 10⁻⁵. By constructing an ICE table with initial and equilibrium concentrations, you substitute values into the Kb expression to solve for hydroxide ion concentration, which then determines pOH and pH of the solution.
Q3: What is the ICE table method and why is it useful for weak base problems?
The ICE table tracks Initial, Change, and Equilibrium concentrations of all species in a weak base equilibrium. It organizes data systematically, showing how concentrations shift from initial values to equilibrium. This method simplifies substitution into the Kb expression and helps identify which concentrations to use when calculating hydroxide ion concentration and subsequent pH values.
Q4: How do you calculate pH from pOH in a weak base solution?
First, calculate pOH by taking the negative logarithm of hydroxide ion concentration. Then use the relationship pH + pOH = 14 to find pH. For example, a 0.23 M ammonia solution with hydroxide ion concentration of 2 × 10⁻³ M yields pOH of 2.70 and pH of 11.30, confirming the basic nature of the solution.
Q5: Can you determine Kb if you know the pH of a weak base solution?
Yes. Convert pH to pOH using pH + pOH = 14, then calculate hydroxide ion concentration from pOH. Construct an ICE table using the known initial concentration and calculated hydroxide ion concentration at equilibrium. Substitute these values into the Kb expression to solve for the base dissociation constant of the weak base.
Q6: Why is the approximation that x is negligible valid in weak base calculations?
Weak bases show partial dissociation, so the change in concentration (x) is typically very small compared to the initial concentration. The approximation is valid when x is less than 5% of the initial concentration. For ammonia, x = 2 × 10⁻³ M is only 0.86% of 0.23 M, justifying the simplification and making calculations manageable.
Q7: What are common applications of weak bases like ammonia?
Ammonia is produced globally in quantities exceeding 100 metric tons annually. It serves as an agricultural fertilizer, a raw material for chemical synthesis of other compounds, and an active ingredient in household cleaners. Its partial dissociation in water and resulting hydroxide ion production make it valuable across industrial, agricultural, and consumer applications.