{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

a3s - Chemistry 2000B Spring 2002 Assignment 3 Solutions 1...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Chemistry 2000B Spring 2002 Assignment 3 Solutions 1. We first have to find the equivalence point: 4 6 8 10 12 pH 0 5 10 15 20 25 30 35 V(OH) (mL) equivalence point pK a When half the acid has been neutralized, the p H equals the pK a . Since the equivalence point is at 18 mL, the pK a is the p H at 9 mL of hydroxide solution added. From the graph, we find pK a 4 8 K a 10 4 8 1 58 10 5 2. The weak base nitrite will react with the strong acid stoichiometrically: NO 2 H HNO 2 We must determine which reagent is in excess. n NO 2 m NaNO 2 M NaNO 2 3 75g 68 9953g mol 0 0544mol n H 0 85mol L 0 065L 0 0553mol The hydrogen ions are in excess by 8 98 10 4 mol. Normally in these problems, the excess protons determine the p H. In other words, we ignore the dissociation of the acid. If we can do this, the concentration of H in solution is just H 8 98 10 4 mol 0 065L 1 38 10 2 mol L 1
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Is it legitimate to ignore the dissociation of the acid? The K a expression for nitrous acid is K a a NO 2 a H a HNO 2 or a NO 2 a HNO 2 K a a H Assuming that a H is approximately 1 38 10 2 (as calculated above), we have a NO 2 a HNO 2 4 5 10 4 1 38 10 2 0 033 In other words, about 3% of the nitrous acid formed will dissociate. The concentration of nitrous acid, again neglecting dissociation, is 0 0544mol 0 065L 0 84mol L so dissocia- tion of the acid would product about 0 027mol L of additional protons. This is bigger than the number of excess protons, so we can’t really neglect this. There are two ways to deal with this problem, both of which revolve around the K a
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}