Chem 222 week 9 notes
Chem 222 week 9 notes Chem 222
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This 2 page Class Notes was uploaded by Leslie Pike on Wednesday March 30, 2016. The Class Notes belongs to Chem 222 at Western Kentucky University taught by Darwin Dahl in Spring 2016. Since its upload, it has received 14 views. For similar materials see College Chemistry 2 in Chemistry at Western Kentucky University.
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Date Created: 03/30/16
How to choose a buffer: the pKa of the buffer should match the pH you desire to buffer at. Sample problem: You mix 5 ml of 0.1 M NaOH with 20 ml of 0.1 M of a weak acid, HA, with a Ka of 10 . What is the resulting pH of this solution? This is a limiting reagents problem. The reaction will go to completion because strong bases such as NaOH react until all is used up. Convert everything to mmol by multiplying molarity by mL. You will get 0.5 mmol NaOH and 2 mmol HA. Use an ICE table. Your limiting reagent is NaOH, as there are fewer moles of it present and species react in a 1:1 mole ratio. Your final concentration of HA is 2mmol-0.5mmol=1.5mmol. Your final concentration of NaA (which will dissolve in solution and be present at A ) is 0.5 mmol. Now, ask yourself: Do I have a buffer in solution? In this case, the answer is yes, because both HA and A- are present in the solution. Since you have a buffer, use the Hendersen-Hasselbalch equation. −¿ A HA ¿ ¿ ¿ pH=pKa−log¿ Is your answer reasonable? You mixed a base and an acid together at the beginning, but you added more acid than base, so your solution will be acidic, so your pH value should be less than 7. Sample problem 2: You mix 20 ml of 0.1 M NaOH with 5 ml of a weak acid, HA, with the same Ka as before. What is the resulting pH of the solution? This is a limiting reagents problem. This reaction will go to completion because of your strong base. Convert everything to mmol by multiplying M by ml. You should get 2 mmol NaOH and 0.5 mmol HA. Use an ICE table. All of your HA will be used up in the reaction; you should end up - with 1.5 mmol NaOH and 0.5 mmol A . Now, ask yourself: Do I have a buffer in solution? In this case, the answer is no, because no HA is present, only A-. You CANNOT use Hendersen-Hasselbalch when you do not have a buffer in solution. What you DO have in solution are two anions, - - OH and A . One is the ion of a strong base and the other is the ion of the weak base. Ignore the weak base and calculate pH using the strong base. −¿ OH ¿ ¿ ¿ pOH=−log¿ pH=14−pOH=12.78 Now ask yourself: Is this answer reasonable? You mixed an acid and a base, and you had more base than acid, so your resulting solution will be basic, so your pH should be above 7. Titrations Titration can be used to determine the concentration of an acid or base. A titration curve is a plot of pH versus amount of titrant added. It is shaped like an “S” with the midpoint of the S being the point at which there is neither acid nor base present in solution, only salt and water. If both the acid and base are strong, your pH will be 7 at the equivalence point. If one is strong and the other is weak, your salt will be acidic or basic and will affect the pH. To tell if a salt is acidic or basic: Strong base strong acid = two neutral ions making a neutral salt. Strong base weak acid = a neutral cation and a basic anion (makes sense that an acid containing a basic anion won’t be as strong of an acid, right?) making a basic salt. Weak base strong acid = an acidic cation and a neutral anion (a base containing an acidic cation will be a weaker base—think aqueous ammonia, NH OH, which + 4 contains the acidic cation NH )4making an acidic salt. Weak base weak acid = if the acid is stronger, you get an acidic salt and vice versa. For example, strontium carbonate is a salt containing strontium and carbonate (well, duh). The strontium would have come from strontium hydroxide, a strong base. Carbonate would have come from carbonic acid, a weak acid (if they are not strong, acids are weak—again, duh). Neutral cation (strontium) and basic anion (carbonate) make a basic salt. To distinguish between base-acid titrations and acid-base titrations: If you have a base and you are titrating it with an acid, your pH will start out high and then decrease. If you have an acid and you are titrating it with a base, your pH will start out low and then increase.
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