Exam III - SP09 - Version A - michel key

Exam III - SP09 - Version A - michel key - 1 Name: _...

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

View Full Document Right Arrow Icon
1 Name: _____________________________ CHEMISTRY 1032 – MICHEL EXAM III – VERSION A Name __________________________________ TUID ____________________ (print clearly) Show all of your work where requested. No graphing or memory accessible calculators are allowed at your examination desk. No cells phones or PDAs are allowed on your persons. Violation of these department rules will be brought to the attention of the Chemistry Department Chairman, the Dean and the Provost, and may result in significant disciplinary action for violation of university standards of academic integrity. Read the problems carefully. Give numerical answers to the correct number of significant figures. The numbers in the [ ] at the end of each question are the point values. Check your work! Useful information is to be found on page 9.
Background image of page 1

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

View Full DocumentRight Arrow Icon
2 Name: _____________________________ 1. Investigators recently obtained a previously unknown acid from the bark of a Peruvian tree. The goal of the investigation was to determine the structure of the acid. As part of this investigation, the titration of the acid with strong base was undertaken. The titration curve for this experiment is depicted below. Use the information contained in the curve to answer the following questions. [20] (a) Is the unknown acid a monoprotic or polyprotic acid? Explain how you can determine this from the shape of the titration curve. [2] monoprotic acid; single S curve (b) The titration curve shows that the unknown acid is a weak acid. Explain how you know this from the characteristics of the titration curve. [2] pH at the equivalence point is >7; pH is set by the conjugate base, A-, A- is a weak base because it comes from a weak acid (c) What is the approximate pK a of the weak acid? Show you determined this from the curve. [3] pKa ~ 5.6. The volume of base required to reach the equivalence point is 12.2 mL. Half that volume is 6.1 mL base, pH at that volume on the curve = 5.6 = pKa 2.00 L
Background image of page 2
3 Name: _____________________________ 1. (continued) (d) Calculate the molarity of the conjugate base, A , before titration with the strong base. Use a pH of 3.3 for the acid solution before titration with the strong base. [3] HA + H 2 O = H 3 O + + A - ; [H 3 O + ] = [A - ] [H 3 O + ] = 10^(-pH) = 10^(-3.3) = 5.01 x 10^(-4)M = [A - ] (e) Calculate the molarity of the weak acid, HA before titration with the strong base. [3] pKa = 5.6 (part 1c), so Ka = 10^(-5.6) = 2.51 x 10^(-6) Ka = 2.51 x 10^(-6) = ] [ ] ][ [ 3 HA A O H - + = (5.01 x 10^(-4))^2/[HA] [HA] = 0.10M (f) Calculate the molar mass of the weak acid. [2] Moles HA = 25.00g/MM = 0.10 mole HA/L x 2.00 L MM = 125.0g (g) Calculate the value of the pH at the equivalence point for the titration. (Hint: what is [A ] at the equivalence point?) [5] Moles A- = 0.20 mole (all HA converted to A-)
Background image of page 3

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

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/26/2011 for the course CHEM 1032 taught by Professor Guenard during the Spring '06 term at Temple.

Page1 / 10

Exam III - SP09 - Version A - michel key - 1 Name: _...

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

View Full Document Right Arrow Icon
Ask a homework question - tutors are online