problem_set_1_key - General Chemistry III Chem-13 Problem...

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General Chemistry III, Chem-13 Problem Set I Key 1. Acetylsalicylic acid (aspirin) whose molecular formula is HC 9 H 7 O 4 , is the most widely used pain reliever and fever reducer. If you have two tablets, each containing 0.325 grams of aspirin (mixed with a neutral “binder” to hold the tablet together), and you dissolve them in a glass of water to give a 225.0 ml solution, what is the pH of the solution? For aspirin use a K a = 3.6 × 10 -4 . Mass of two tablets = 2 × 0.325 grams = 0.650 grams Molar mass of aspirin = 180 grams/mol Number of moles of aspirin = mole grams grans 180 650 . 0 = 0.00361 Molarity of aspirin solution = L moles 225 . 0 00361 . 0 = 0.0160 M HA H + + 0.0160 0 0 -x x x 0.0160-x x x 4 2 10 6 . 3 0160 . 0 - × = - = x x K a Solve for x using quadratic equation: x = 0.0022 pH = 2.65 If you did not use the quadratic equation: x = 0.0024 pH = 2.61
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2. The base ethylamine (CH 3 CH 2 NH 2 ) has a K b of 4.3 × 10 -4 and the base ethanolamine (HOCH 2 CH 2 NH 2 ) has a K b of 3.2 × 10 -5 . a. Which of the two bases is stronger? How did you arrive at your answer? B + H 2 O BH + + OH¯ K b = ] [ ] ][ [ B OH BH - + A stronger base releases more OH¯ in solution. If the [OH¯] is larger then the K b value will be larger. Therefore ethylamine is the stronger base of the two as it has a higher value of K b . b. Calculate the pH of a 0.10 M solution of ethylamine. B + H 2 O BH + + OH¯ 0.10 – x x x K b = 4 2 10 3 . 4 10 . 0 - × = - x x x = 0.0066 M pOH = 2.18 and therefore pH = 11.8 c. Calculate the pH of a 0.10 M solution of ethanolamine. B + H 2 O BH + + OH¯ 0.10 – x x x K b = 5 2 10 2 . 3 10 . 0 - × = - x x x = 0.00179 M pOH = 2.74 and therefore pH = 11.3
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3. The local anesthetic novocaine is the hydrogen chloride salt of an organic base, procaine. C 13 H 20 N 2 O 2(aq) (Procaine) + HCl (aq) [HC 13 H 20 N 2 O 2 ] + Cl¯ (aq) (Novocaine) The pK a for novocaine is 8.85. What is the pH of a 0.0015 M solution of novocaine? Novocaine is an acidic salt and lets represent it as NoCl NoHCl NoH + + Cl¯ 0.0015 0.0015 0.0015 pH will only depend on the No + (Because Cl¯ is the conjugate base of a strong acid, it does not contribute significantly to the pH).
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