…a lesson in resistance Pretend that you are making a M solution of benzoic acid. Another name for benzoic acid is _______________ hydrogen benzoate Hydrogen benzoate is what kind of acid? _________________ An organic acid Write the acid equilibrium reaction between benzoic acid and the water into which it’s dissolved. Determine the [H 3 O + ]

…a lesson in resistance X = [ H O ] = You know the next task…. pHind the pH !!! The pH of this weak acid solution is 2.16

…a lesson in resistance Name a salt of the conjugate base that could be added to the benzoic acid that would not add to the acidic nature of the solution…. One possible answer would be NaC 7 H 5 O 2 If you added solid sodium benzoate to the solution such that the concentration of the hydrogen benzoate and the sodium benzoate turned out to 0.750M each, what major species would be present?

…a lesson in resistance HC 7 H 5 O 2,Na +,C 7 H 5 O 2 -,H 2 O What would the concentrations of these species be, that are of any importance, before any reaction occurs? 0.750M Doesn’t matter, won’t influence things M, you can neglect the contribution from the weak acid. Doesn’t matter, won’t influence things What reaction contains both of the major species identified?

…a lesson in resistance Once again, pHind the pH _____ K a equals the hydronium ion concentration…i.e. [H 3 O + ] = 6.4 x

…a lesson in resistance Find the pH of a solution to which 1.0 mL of 6.0 M HCl was added to mL of water. Hints: 1. HCl is strong acid 2. Find the number of moles of HCl present…that’s the same as the H 3 O + ion 3. Divide the number of moles of H 3 O + by the new volume it is in to find [H 3 O + ]

…a lesson in resistance What is the pH of the water with the 1.00 mL of HCl? ____ Did the water resist a change in pH with the addition of a small amount of acid? ____ What is the pH of pure water? _____ No !!!

…a lesson in resistance If you were to add 1.0 mL of 6.0 M HCl to 250 mL of the solution you previously made from the benzoic acid and sodium benzoate, what would happen to the pH?

…a lesson in resistance Always do the stoichiometry first… moles of HCl will react with moles of benzoate in the following reaction ( 1 to 1 mole ratios): Ask yourself, what is the HCl most likely to react with that is present as a major species (hint: look for the strongest base around) _____________ Therefore, moles of benzoic acid is produced as well, as this is a reaction with a strong acid and will go to completion. The benzoate ion HCl + C 7 H 5 O > HC 7 H 5 O 2 + Cl -

…a lesson in resistance HC 7 H 5 O 2 + H 2 O H 3 O + + C 7 H 5 O 2 - (0.773 M) (0.725 M)

…a lesson in resistance Once again, phind the pH _____ You have easily found the hydronium ion concentration to be 6.8 x

…a lesson in resistance Did the solution resist a change in pH with the addition of a small amount of acid? _________________ The answer is …a resounding yes!!! What was the pH of the solution after you added the acid? ____ What was the pH of the solution before you added the acid? ____

…a lesson in resistance “What is it that a solution of a weak acid and a salt of its conjugate base is able to do upon the addition of a small amount of even a strong acid? The answer is …resist a change in its pH… something water could not do!!!!! I guess you know that a solution that can do that is called a “buffer”.

…a lesson in resistance What is the pH of a buffer solution that is 1.25 molar HNO 2 and KNO 2 respectively?

…a lesson in resistance HNO 2(aq) + H 2 O (l)  H 3 O + (aq) + NO 2 - (aq) Init: 1.25 M ~ M change: -x +x +x Eq: 1.25 M-x x 1.25 M+x Ka = [x] [ x ] = 4.0 x [ 1.25-x ] [x] = 4.0 x pH = 3.40 What if you had 50.0 mL of the buffer and added 2.00 mL of 2.00 M HNO 3 to it…what would the pH become as a result?

…a lesson in resistance Original Question: What if you had 50.0 mL of the buffer and added 2.00 mL of 2.00 M HNO 3 to it…what would the pH become as a result? Always do the stoichiometry first! Moles of HNO 3 (a strong acid): ( L)(2.00 mol/liter)= mol HNO 3 Moles of HNO 2 (the weak acid in the buffer): (0.0500L)(1.25 mol/liter)= mol HNO 2 Moles of NO 2 - (the [conj] base in the buffer): (0.0500L)(1.25 mol/liter)= mol NO 2 -

…a lesson in resistance Original Question: What if you had 50.0 mL of the buffer and added 2.00 mL of 2.00 M HNO 3 to it…what would the pH become as a result? Always do the stoichiometry first! The strong acid will seek the strongest major species base that is present… Therefore it will react with the NO 2 - : This reaction goes to completion because the hydronium really wants to give up its proton!!! Hence, the nitrite goes down by the number of moles of strong acid added and the nitrous acid goes up by the number of moles of strong acid added NO 2 - (aq) + H 3 O + (aq) -> H 2 O (aq) + HNO 2 (aq)

…a lesson in resistance Original Question: What if you had 50.0 mL of the buffer and added 2.00 mL of 2.00 M HNO 3 to it…what would the pH become as a result? NO 2 - : mol – mol = mol HNO 2 : mol mol = mol Each of these moles now exist in 52.0 mL of solution (the original 50.0 mL and the extra 2.00 from the strong acid addition) Therefore: [NO 2 - ] = mol/0.052 L = 1.13 M [HNO 2 ] = mol/0.052 L = 1.28 M

…a lesson in resistance HNO 2(aq) + H 2 O (l)  H 3 O + (aq) + NO 2 - (aq) Init: 1.28 M ~ M change: -x +x +x Eq: 1.28 M-x x 1.13 M+x Ka = [x] [ x ] = 4.0 x [ 1.28-x ] [x] = 4.5 x pH = 3.35 Did the addition of the strong acid make the pH of the solution of the weak acid and salt of its conjugate base change much? Now do the equilibrium problem NO! Previous pH = 3.40

…a lesson in resistance If a buffer can be made from a weak acid and a salt of its conjugate base then what other scenario can you envision? …a weak base and a salt of it’s conjugate acid. Like… ammonia and ammonium chloride in solution.

…a lesson in resistance Find the pH of the buffer solution containing 0.50 M NH 3 and NH 4 Cl. Then find the pH of the solution after 1.0 mL of 0.75 M NaOH was added to mL of it. Also, find the pH of mL of pure water after the same addition (1.0 mL of NaOH)

…a lesson in resistance NH 3(aq) + H 2 O (l)  OH - (aq) + NH 4 + (aq) Init: 0.50 M ~ M change: -x +x +x Eq: 0.50-x x 0.50 M+x Kb = [x] [ x ] = 1.8 x [ 0.50-x ] [x] = [OH - ]= 1.8 x pOH = 4.74 pH = 9.26 Now find the pH of the solution after 1.0 mL of 0.75 M NaOH was added to mL of this solution. First Initial Question: Find the pH of the buffer solution containing 0.50 M NH 3 and NH 4 Cl.

…a lesson in resistance Always do the stoichiometry first! Moles of NaOH (a strong base): ( L)(0.75 mol/liter)= mol NaOH Moles of NH 3 (the weak base in the buffer): (0.100L)(0.50 mol/liter)= mol NH 3 Moles of NH 4 + (the [conj] acid in the buffer): (0.100L)(0.50 mol/liter)= mol NH 4 + Second Question: Find the pH of the solution after 1.0 mL of 0.75 M NaOH was added to mL of the 0.50 M buffer of NH 3 /NH 4 Cl.

…a lesson in resistance Always do the stoichiometry first! The strong base will seek the strongest major species acid that is present… Therefore it will react with the NH 4 + : This reaction goes to completion because the hydroxide really wants to take a proton!!! Hence, the ammonium goes down by the number of moles of strong base added and the ammonia goes up by the number of moles of strong base added NH 4 + (aq) + OH - (aq) -> H 2 O (aq) + NH 3 (aq) Original Question: Find the pH of the solution after 1.0 mL of 0.75 M NaOH was added to mL of the 0.50 M buffer of NH 3 /NH 4 Cl.

…a lesson in resistance Always do the stoichiometry first! Now you can find the new concentrations! NH 4 + : mol – mol = mol NH 3 : mol mol = mol Each of these moles now exist in mL of solution (the original mL and the extra 1.00 from the strong base addition) Therefore: [NH 4 + ] = mol/0.101 L = 0.49 M [NH 3 ] = mol/0.101 L = 0.50 M Original Question: Find the pH of the solution after 1.0 mL of 0.75 M NaOH was added to mL of the 0.50 M buffer of NH 3 /NH 4 Cl.)

…a lesson in resistance Init: 0.50 M ~ M change: -x +x +x Eq: 0.50 M-x x 0.49 M+x K b = [x] [ x ] = 1.8 x [ 0.50-x ] [x] = [OH - ]= 1.8 x pOH = 4.74 Now do the equilibrium problem NH 3(aq) + H 2 O (l)  OH - (aq) + NH 4 + (aq) pH = 9.26 No appreciable change!

…a lesson in resistance Moles of NaOH (a strong base): (0.0010L)(0.75 mol/liter)= mol OH mol OH - / L = [OH - ] M = [OH - ] Final Question Also, find the pH of mL of pure water after the same addition (1.0 mL of NaOH) pOH= 2.13 pH= a significant change from pH = 7 in pure water!