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HW3_14

HW3_14 - Homework due 3/14 Calculate the pH of 0.20 L of an...

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Homework due 3/14 Calculate the pH of 0.20 L of an NH 3 -NH 4 NO 3 buffer that is 0.40 M in NH 4 NO 3 and 0.50 M in NH 3, a) initially, b) after 0.050 mole of HNO 3 have been added, c) after 0.040 moles KOH have been added to a fresh 0.20 L of the buffer. (K b for NH 3 = 1.8x10 -5 ) a) initially The first problem is to simply calculate the pH of the buffer as given. This can be done by substituting into the Henderson-Hasselbalch equation, where the acid form is NH 4 1+ (from the NH 4 NO 3 ), and the base form is NH 3 . NH 4 NO 3 NH 4 1+ + NO 3 1- {NO 3 1- is a spectator ion since it is from the strong acid HNO 3 } NH 3 + H 2 O NH 4 1+ + OH 1- or NH 4 1+ NH 3 + H 1+ pOH = pK b + log ([NH 4 1+ ]/[NH 3 ]) = -log 1.8x10 -5 + log (0.40/0.50) = pOH = 4.74 + (-0.10) = 4.64 pH = 14 - pOH = 14 - 4.64 = 9.36 Alternatively, pH = pK a + log [NH 3 ]/[NH 4 1+ ] pK a = 14 - pK b = 14 - 4.74 = 4.26 pH = 9.26 + log (0.50/0.40) = 4.26 + 0.10 = 9.36 b) After the addition of 0.050 mole HNO 3 . The H

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HW3_14 - Homework due 3/14 Calculate the pH of 0.20 L of an...

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