Strong Acid/Base Calculations

Calculating Ion Concentrations for Strong Acids & Bases For strong acids and bases, the concentration of the ions can be readily calculated from the balanced equation. Since strong acids and bases ionize/dissociate COMPLETELY, we can assume that all of the substance changes into aqueous ions Determine the balanced ionization/dissociation equation Use the coefficients from that equation to do the stoichiometry calculation Consider the following examples

Calculating Ion Concentrations for Strong Acids & Bases 1. Calculate the hydronium/hydrogen ion concentration in a 0.050 M solution of hydrochloric acid. Solution: We know that HCl is a strong acid that ionizes completely in water (you should memorize the list of strong acids). Begin by writing the balanced reaction: HCl(aq) + H2O (l) → H3O+(aq) + Cl-(aq) From the balanced equation we see that 1 mole of HCl produces 1 mole of H3O+ (a 1:1 ratio), therefore the concentration of H3O+ will equal that of HCl. Answer: [H3O+ ] = 0.050 M Also [Cl-] = 0.050 M

Calculating Ion Concentrations for Strong Acids & Bases 2. Calculate the hydroxide ion concentration in a 0.010 M solution of barium hydroxide, Ba(OH)2. Barium hydroxide is a strong base.   Solution: Always begin by writing a balanced equation: Ba(OH)2 (aq) → Ba2+(aq) + 2 OH-(aq) Since 2 moles of OH- are produced for every 1 mole of Ba(OH)2 , the concentration of OH- will be twice the concentration of Ba(OH)2 . Answer: [OH-] = 2 × 0.010 = 0.020 M Also [Ba2+] = 0.010 M

Weak Acid/Base Calculations

Calculating Ion Concentrations for Weak Acids & Bases Weak acids and bases require a much different approach to finding ion concentrations. Once you know you have a weak acid or base, follow these steps in finding ion concentrations: Write a balanced equation for the reaction You will need to know the value of Ka or Kb - if it is not given in the question, look it up in a Table of Acid and Base Strengths. Set up the equilibrium constant expression. Typically you will know the value of Ka (or Kb) and the concentration of the acid; you will be solving the equation for the concentration of the ions.

Calculating Ion Concentrations for Weak Acids 3. Calculate the hydrogen/hydronium ion concentration in a 0.10 M acetic acid solution, HC2H3O2. Ka for acetic acid, a weak acid, is 1.8 ×10-5. Begin by writing the balanced reaction: HC2H3O2 (aq) + H2O (l) ↔ H3O+(aq) + C2H3O2-(aq) The question gives us the concentration of the acid, HC2H3O2 (0.10 M). PLEASE NOTE: ionization is NOT complete because this is a weak acid , [H3O+] will NOT equal [HC2H3O2]. We CANNOT use stoichiometry - Instead we must calculate it using the equilibrium constant expression.

Calculating Ion Concentrations for Weak Bases 4) Calculate the hydroxide ion concentration, [OH-], in a 0.025 M solution of analine, C6H5NH2, a weak base with Kb = 4.3×10-10 Begin by writing a balanced equation. Since analine is a base that doesn't contain the hydroxide ion, include H2O as a reactant. NOTE: ALWAYS INCLUDE WATER FOR WEAK BASES! Also remember that a base is a hydrogen acceptor and will gain an additional H+: C6H5NH2 (aq) + H2O (l) ↔ C6H5NH3+(aq) + OH-(aq) As we did in the previous example, we now set up the Kb expression and solve for ion concentrations. We see from the balanced equation that the ions have a 1:1 ratio, therefore [OH-] will equal the [C6H5NH3+].