CHM1046 Study Guide for Exam 4 16 1 The Danger of AntiFreeze probably won t need to know If you re in a hurry skip this if you have time it s worth looking over Blood maintains a pH between 7 36 7 42 This is essential because most proteins in the body require a very narrow pH range in order to function Because of this pH requirement our blood contains buffer systems to resist change in pH and ensure all proteins work properly If excess antifreeze is consumed it will overwhelm the buffer system causing the pH to drop to dangerous levels often resulting in death 16 2 Buffers Solutions that Resist pH Change buffer a substance that resists pH change by neutralizing added acid of added base contain large amounts of either a weak acid and its conjugate base or a weak base and it s conjugate acid Therefore when an acid is added to a solution the conjugate base reacts with the acid to neutralize the solution or when a base is added to a solution that weak acid reacts with the base to neutralize the solution Calculating the pH of a Buffer Solution step 1 write out the balanced equation for the ionization of the acid step 2 set up an ICE table step 3 solve for the concentrations by using the Ka value step 4 solve for the pH Henderson Hasselbalch Equation Calculating pH Changes in Buffer Solutions While buffers are used to resist pH change they do NOT eliminate pH change The Stoichiometric Calculation As acid is neutralized a stoichiometric amount of the base is converted to its conjugate acid through the neutralization reaction EX In order to neutralize 025mol of a strong acid H 025 mol of the weak base A is required Therefore the amount of A decreases by 025 mol and the amount of HA increases by 025 mol We track these changes in a chart sort of like an ICE table After the changes have been tracked we can then use an ICE table to solve for the final pH by using the steps listed above for calculating the pH of a buffer solution 16 3 Buffer Effectiveness Buffer Capacity and Buffer Range Buffers are most resistant to pH change when the concentrations of acid and conjugate base are equal The relative concentrations of acid and conjugate base should not differ by more than a factor of 10 Buffers are most resistant to pH change when the concentrations of acid and conjugate base are highest Buffer Range In order to determine the effective range for a buffering system which is one pH unit on either side of the pKa you solve for the lowest pH and the highest pH based on the fact that the ratio should not differ by more than a factor of 10 Buffer Capacity Buffer Capacity the amount of acid or base that can be added to a buffer without destroying its effectiveness We can conclude from what we learned earlier about the effectiveness of buffers that Buffer capacity increases as the concentrations of buffer components increase Buffer capacity increase as the relative concentrations of buffer components become closer to eachother 16 4 Titrations and pH Curves acid base titration a basic or acidic solution of unknown concentration is reacted with an acidic or basic solution of known concentration The known solution is added to the unknown while the pH is measured As the two combine they neutralize eachother Equivalence point the point in the titration when the number of moles of the base is stoichiometry equal to the number of moles of the acid the titration is complete The titration of a strong acid with a strong base EX 25mL of 1M HCl is titrated with 1M of NaOH Calculate the volume of the base required to reach the equivalence point step 1 write out the chemical equation step 2 calculate the number of moles in the initial substance step 3 convert the number of moles of the titrant into volume to determine the answer Calculating the pH of titration of a strong acid with a strong base step 1 write out the neutralization reaction step 2 Calculate the initial pH before any base is added step 3 Calculate the pH after adding 5mL of NaOH step 4 Calculate the pH after adding 10 15 20 mL of NaOH step 5 Calculate the pH after adding 25 mL of NaOH this will be the pH at the equivalence point for this problem NOTE the pH for a strong acid strong base reaction will always by 7 BUT that is not true for other types of titrations step 6 after adding 30mL of NaOH more than the equivalence point the base will be in excess and therefore the pH will be much higher This is calculated in the same way as steps 3 4 you just have to convert the pOH to the pH as done in the past unit The overall pH curve for a strong acid strong base titration The Titration of a Weak Acid with a Strong Base Consider the titration of 25 0mL of 1M HCHO2 with 1M NaOH step 1 write out the chemical equation step 2 compute the volume required to reach the equivalence point of the titration step 3 calculate the initial pH note since it is not a strong acid you must do this by making an ICE chart for the equilibrium reaction step 4 calculate the pH after adding 5mL NaOH step 5 calculate the pH after adding 10 12 5 15 20mL NaOH step 6 calculate the pH after adding 25 mL the equivalence point for this problem At this point the solution is no longer a buffer but rather just an ion You can see this because the only substance remaining in the solution from step 6 is the CHO2 therefore we calculate the pH by solving an equilibrium problem involving the ionization of water by the weak base Titration Curve for a Weak Acid with a Strong Base Titration of a Polyprotic Acid When a diprotic acid is titrated with a strong base and if Ka1 and Ka2 are different the pH curve will have TWO equivalence points The first equivalence point represents the titration of the first proton while the second equivalence point represents the titration of the second proton NOTE the volume required to reach the first equivalence point is the same as the volume required to reach the second 16 5 Solubility Equilibria and the Solubility Product Constant Solubility Product Constant the equilibrium expression for a chemical equation representing the dissolution of an ionic compound solubility the quantity of the compound that dissolves in a certain amount of liquid molar solubility the solubility in units of mol L Calculating the molar solubility from KsP step 1 write the reaction step 2 prepare an ICE tbale step 3 substitute the equilibrium expressions into the expression for Ksp step 4 solve for S EX Calculate the molar solubility of PbCl2 in pure water Ksp and Relative Solubility Ksp
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