TUTORIAL 10 Calculating ph of Weak Acid Solutions Percent Ionization Solutions of Bases Acid-Base Properties of Ions and Salts CALCULATING THE ph OF WEAK ACID SOLUTIONS Tro - Sections 15.6 to 15.8 FORMAL CONCENTRATION If we make a solution that is 0.20 M HF, the concentration of HF in the solution will not really be 0.20 M since some of the HF dissociates to form H3O + and F - Nevertheless, chemists usually refer to the concentration of weak acids solutions as if none of the acid had dissociated This concentration is called the formal concentration The extent to which a weak acid dissociates is reflected in its Ka value. HA(aq) + H2O(l) A - (aq) + H3O + (aq) Remember... the higher the Ka value, the more the equilibrium favours the products The acid is more highly dissociated The stronger the acid PERCENT DISSOCIATION (OR IONIZATION) We can quantify the ionization of a (monoprotic) weak acid, HA, using a simple ratio: Percent Ioniztion = [H 3O + ] equi [HA] initial 100 For a weak acid, the lower the concentration, the higher the percent ionization. A TYPICAL PROBLEM Given: The formal concentration of an acid solution Acid dissociation constant, Ka Asked to find: The [H3O + ], ph of the solution, or the percent dissociation
SAMPLE PROBLEM (A) VALIDITY OF ASSUMPTIONS The x is small assumption used in the previous problem made the calculation a lot easier. Phenol (C6H5OH) is a weak acid used as a general disinfectant and in the manufacture of plastics. Calculate the ph and the percent dissociation of a 0.10 M solution of phenol if the Ka of phenol is 1.3 x 10-10. The assumption is only valid when: The formal concentration of the acid is large compared to the dissociation constant The ratio of x to the formal concentration is less than 5%. If we find the assumption is not valid, we must go back and do the calculation again, solving the quadratic equation. SAMPLE PROBLEM (B) Formic acid (HCO2H) is a weak organic acid secreted by ants and stinging nettles. It has a dissociation constant of 1.8 x 10-4 Calculate the ph and the concentration of all species present in 0.020 M formic acid solution. SOLUTIONS OF BASES STRONG BASES Like strong acids, strong bases fully dissociate, like NaOH NaOH(aq) Na + (aq) + OH - (aq) The [OH - ] in solution can be easily calculated if we know the concentration of strong base. The ph of the solutions can be calculated by first calculating the poh of the solution. Remember... ph + poh = 14
SAMPLE PROBLEM (C) WEAK BASES Weak bases, like weak acids, do not fully dissociate in aqueous solution They accept protons from water, forming OH - ions in solution: Calculate the ph of a 0.0100 M solution of calcium hydroxide, Ca(OH)2 B(aq) + H2O(l) OH - (aq) + BH + (aq A base dissociation constant, Kb, quantifies the extent of dissociation of the base Again, like weak acids, the higher a base s Kb value, the stronger the base SAMPLE PROBLEM (D) Morphine, used in hospitals as a strong pain reliever, is a weak base. It has a Kb of 1.6 x 10-6 at 25 o C. A solution of morphine has a concentration of 0.22 M. 1. What is the ph of the solution? 2. Calculate the percent ionization of morphine in the solution. Anions and cations in ionic compounds can act as acids an bases. ACID-BASE PROPERTIES OF IONS AND SALTS When these compounds are dissolved in water they sometimes make the solution acidic or basic. We must learn about the relative strengths of conjugate acid-base pairs in order to predict whether a salt solution will be acidic, basic or neutral.
ANIONS AS WEAK BASES EXAMPLES Most anions can be thought of as conjugate bases of weak acids. For example NO2 - is the conjugate base of the weak acid HNO2 In general... If the anion is the conjugate of a weak acid, it is a weak base If the anion is the conjugate of a strong acid, it is ph-neutral F - is the conjugate base of HF. We know that HF is a weak acid. That makes F - a weak base: F - (aq) + H2O(l) OH - (aq) + HF(aq) NO3 - is the conjugate base of HNO3. HNO3 is a strong base, making NO3 - ph-neutral. It will not ionize in aqueous solution. NO3 - + H2O No Reaction Cations can be grouped into one of three categories 1. Counterions of strong bases 2. Conjugates of weak bases 3. Small, highly charged (> 3+) metal ions In general... CATIONS AS WEAK ACIDS Counter ions of strong bases are ph-neutral Conjugates of weak bases are weak acids Small, highly charged metal ions become hydrated and are weak acids EXAMPLES Ca 2+ is the counterion of Ca(OH)2, a strong base. Ca 2+ is therefore ph-neutral: Ca 2+ (aq) + H2O(l) NH4 + is the conjugate acid of NH3, a weak base. This will make NH4 + a weak acid: NH4 + (aq) + H2O(l) Al 3+ will become hydrated in solution forming Al(H2O)6 3+. This hydrated species acts like a weak acid: Al(H2O)6 3+ (aq) + H2O(l) No Reaction NH3(aq) + H3O + (aq) Al(H2O)5OH 2+ (aq) + H3O + (aq) PREDICTING THE ph OF SALT SOLUTIONS SAMPLE PROBLEM (E) Consider the anion and cation of the in the salt separately: Predict whether each is a weak acid/base or ph-neutral. If one ion is neutral and the other is a weak base, the solution will be basic If one ion is neutral and the other is a weak acid, the solution will be acidic If one is a weak base and the other is a weak acid, we cannot predict the ph of the solution without knowing the Ka and Kb values. Predict the ph of aqueous solutions of the following salts: 1. NaBr 2. K2SO3 3. NH4Cl 4. Cr(NO3)3
THE RELATIONSHIP BETWEEN Ka and Kb FOR CONJUGATE PAIRS SOLUTIONS CONTAINING AN ACIDIC CATION AND BASIC ANION For a conjugate acid-base pair, the following equation applies (at 25 o C): Ka Kb = 1.0 x 10-14 This relationship is useful if we need to know the dissociation constant of a species and are given the dissociation constant of its conjugate. If the solution contains both a cation that is a weak acid and an anion that is a weak base, we must compare the Ka of the acid with the Kb of the base. The species with the larger K value will dominate as this species will be a stronger acid or base than the other species. SAMPLE PROBLEM (F) Predict whether an aqueous solution of NH4F is acidic, basic, or neutral. The Kb of NH3 is 1.75 x 10-5 and the Ka of HF is 3.5 x 10-4.