{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

24 Buffers

24 Buffers - 33.3 mL of 0.2 M HCl is added to For E and EH...

This preview shows pages 1–4. Sign up to view the full content.

1 1 33.3 mL of 0.2 M HCl is added to 25.0 mL of a 0.3 M solution of ephedrine . What is the pH of the resulting solution? A) 8.31 B) 6.06 C) 7.00 D) 9.06 E) 9.96 For E and EH + K b = 9.1x10 -5 K a = 1.1x10 -10 9.96 was the pH before the acid was added, note how little the pH changed! Why? 1. Determine millimoles of E present initially. 2. Determine millimoles of HCl added. 3. Determine millimoles of EH + produced. 4. Determine millimoles of E remaining. 5. Use K a to calculate the hydronium ion concentration and hence the pH. The base E consumed the strong acid to make a weak acid! 2 Construct a RICE table for HCl reacting with E. Initial amount of HCl and E: HCl: (0.2 M x 33.3 mL) = 6.66 mmoles E: (0.3 M x 25.0 mL) = 7.50 mmoles HCl + E EH + + Cl - 6.66 7.50 0 0 mmoles -6.66 -6.66 +6.66 +6.66 0 0.84 +6.66 +6.66 Major species: H 2 O, E, EH + , Cl Equilibria involving E, EH + , and H 2 O determine the pH. Chloride is a very very weak base and isn’t involved. It is the conjugate base of a strong acid. Strong acid reacting with a base. Reaction goes to completion.

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

View Full Document
2 3 Since the pH is requested, work with the acid ionization equilibrium. EH + + H 2 O E + H 3 O + ] [EH ][E] O [H K 3 a In terms of concentration, volume = 25.0 mL + 33.3 mL 6.66/58.3 mL 0.84/58.3 mL 0 -x x x 6.66/58.3-x 0.84/58.3+x x HCl + E EH + + Cl 6.66 7.50 0 0 mmoles -6.66 -6.66 +6.66 +6.66 0 0.84 +6.66 +6.66 4 Equilibrium concs in M EH + + H 2 O E + H 3 O + 6.66/58.3-x 0.84/58.3+x x For E and EH + K b = 9.1x10 -5 K a = 1.1x10 -10 x) - (6.66/58.3 x) 8.3 (x)(0.84/5 10 1.1 ] [EH ][E] O [H K 10 - 3 a x 9.06 pH 0.84 6.66 10 1.1 x 6.66 0.84 x 10 1.1 10 10 x x 25.0 mL + 33.3 mL = 58.3 mL Shortcut!
3 5 The Buffer Region in an Acid – Base Titration The region where both a weak acid and a conjugate base are major species in the solution.

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

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

Page1 / 10

24 Buffers - 33.3 mL of 0.2 M HCl is added to For E and EH...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online