WCLN - Strong Acid-Weak Base Titration Curves - Chemistry

This video shows how a titration curve is constructed using data from the titration of a weak base with a strong acid. It also summarizes some of the important features of this Strong Acid-Weak Base titration curve, such as the buffer region and the pH at the equivalence point. http://www.BCLearningNetwork.com. 0:00a titration curve is a graph which shows 0:07how the ph of an acid solution changes 0:09as a base is added to it or how the ph 0:13of a base Aleutian changes as an acid is 0:15added to it here will consider the 0:18addition of a strong acid to a solution 0:20which is initially a weak base the 0:23strong acid will use in our example is 0:25point one molar HCL and the weak base 0:29will use this point 1 molar nh3 we have 0:32initially added 25 milliliters a point 1 0:35molar nh3 to the beaker the ph meter 0:38will be used to monitor the ph of the 0:40mixture in the beaker below the beer at 0:43what we'll do is draw a graph of the ph 0:46and the beaker versus the volume of HDL 0:49added to the NH 3 in the beaker 0:52we'll start with the point 1 molar and 0:54h3 solution in the beaker no HCL acid 0:58has been added yet 1:00nh3 is a weak base so the initial ph 1:03will be above seven 1:06it could be determined that the ph a 1:08point 1 molar nh3 is equal to 11.1 to 1:12this is where the curve starts 1:15as the first three milliliters of HCl is 1:18added the ph goes down very quickly 1:25from three milliliters 1:26224 milliliters of HCL added the nh3 is 1:30an excess and the HCL is the limiting 1:33reagent the limiting reagent HCL will 1:37react with some of the excess and h3 to 1:40form some nh4 plus and Cl minus 1:44because HCL is the limiting reagent it 1:47will all be used up 1:51and co- is a spectator so what does not 1:53affect pH so we'll discard its formula 2:01the HCL 2:03back with some of the excess and h3 and 2:06we'll be left with less than we started 2:08with 2:12so during this portion of the titration 2:13we have some weak base left over 2:17but we have also formed some of its 2:19conjugate acid nh4 plus 2:22recall that a mixture of a weak base and 2:25it's conjugate acid forms a buffer 2:27solution 2:29a buffer solution minimizes the change 2:32in pH as HCL is added to the mixture in 2:35the beaker 2:37so because of this buffering effect 2:38between three mills and 22 miles the 2:42slope of the curve is less steep 2:50as we go from 20 to milliliters to 25 2:53milliliters of HCL added the buffer 2:55solution is overcome and the ph falls 2:58deeply 3:03at 25 milliliters of HCL added we have 3:06reached the equivalence point of this 3:08titration in order to understand what we 3:11have at the equivalence point we 3:13construct what is called an ICF table 3:17i stands for the initial moles C stands 3:20for the change in the number of moles as 3:22the reaction goes to completion and f 3:25stands for the final number of moles of 3:27each component remaining 3:29initially we had 25 milliliters of point 3:321 molar nh3 in the beaker this is point 3:35zero two five liters times point 1 mole 3:38per liter which equals point 0025 mold 3:42at the equivalence point we added 25 3:44milliliters 2.1 molar HCL which is also 3:48a point 0 0 25 mold 3:51if we imagine a time just before the 3:53reaction starts we have no products yet 3:57HDL is a strong acid so this reaction 4:00goes to completion in the process point 4:030 0 25 moles of both nh3 and HCL are 4:07consumed 4:09and 0 moles of these two reactants 4:11remain after the reaction 4:14according to stoichiometry 8.00 25 moles 4:18of NH 3 and HCL react point 0 0 25 moles 4:22of both nh4 plus and Cl minus will be 4:26formed 4:27so when the reaction is complete we will 4:30have 0 plus point zero zero two five 4:32equals point 0 0 25 moles of both nh4 4:36plus and Cl minus because the CL minus 4:40sign is the conjugate base of the strong 4:42acid HCl it is a spectator ion and will 4:46not affect the pH so will eliminate that 4:49from our table 4:51once the reaction at the equivalence 4:53point is complete there is no longer any 4:56nh3 or HCL present 4:59so will also eliminate these former 5:02cable of what is present at the