Presentation on theme: "Only a portion of the weak acids and bases will break apart when in H 2 O Only a fraction of the molecules will create H + or OH - Use the Keq to determine."— Presentation transcript:
Only a portion of the weak acids and bases will break apart when in H 2 O Only a fraction of the molecules will create H + or OH - Use the Keq to determine how much of the chemical breaks up HF (aq) H + (aq) + F - (aq) Keq = [H + ][F - ] [HF] This Keq = Ka Ionization constant for acids Higher Ka value = more breaking apart of the acid = more H + in water = STRONGER ACID
2. Calculating Ka from pH If we know the pH, we can get the Ka 1. Use the equilibrium chart 2. Use pH to determine [H + ] at equilibrium 3. Determine other concentrations at equilibrium 4. Use Ka expression to calculate Ka Example If the pH of a 0.1 M HX is 4.75, what is Ka? HX H + + X M 0.0 M -x X X 1 x x x The subtracted amount is so small, we can ignore it!!
Ka = [H + ][X - ] [HX] 3. Determining the Percent ionized acid If we need to know what percent of the acid is ionized % =. [H + ]. original concentration Percent ionized = x x Ka = (1.778 x ) Ka = 3.16 x %
Example – Calculate the Ka of a 0.50 M solution of formic acid (HCHO 2 ) with a pH of Also determine the percent ionization in the acid. HCHO 2 H + + CHO M 0.0 M -x X X 1 x x – 9.55 x Again, the subtracted amount is so small, we can ignore it!! Ka = (9.55 x ) Ka = 1.8 x Ka = [H + ][CHO 2 ] [HCHO 2 ] Percent ionized = [H + ] original [HCHO 2 ] 9.55 x %
4. Finding pH from Ka Done much the same way 1. Set up the chart 2. Determine final concentration in terms of x 3. Us Ka to solve for x and final [ ] of all products 4. Take the –log of the final [H + ] Example – If HY has a Ka of 5.3 x 10 -5, what is the pH of a 0.1 M solution? HY H + Y M 0.0 M -x X X x x x But, in previous problems, 0.1 – x is essentially 0.1 If ionization is less than 5%, we can ignore the amount of ionized acid from the original amount
When we finish the problem, we should check to make sure that the ionization is less than 5%. Ka = X x = X X = 2.30 x This means that [H + ] is 2.30 x M pH = -log (2.3 x ) pH = 2.64 Now check to make sure ionization is less than 5% % ionized = [H + ] x 100 [HY] 2.30 x x % We’re safe!