Experiment 3: Titration As An Analysis Tool - The Identification Of An Unknown Solid Acid

Question 1

The Bronsted and Lowry Acid and Base Theory

Answer 1

An acid is a substance that can donate protons, a base is a substance that can accept a proton and neutralization is a proton transfer from acid to base

Question 2

The acid strength of a Bronsted-Lowry acid:

Answer 2

Refers to the ability of an acid to relinquish protons
- e.g. a strong acid gives up protons more readily compared to a weak acid
- if an acid is strong and easily loses protons, its conjugate base does not readily accept protons
- thus, the strength of an acid is inversely related to the strength of its conjugate base

Question 3

Weak Acids vs Strong Acids

Answer 3

• Strong acids are fully ionized in an aqueous solution
• weak acids are only partially ionized
  E.g. an aqueous solution of HCl (a strong acid) contains primarily H3O+ and Cl- ions, however an aqueous solution of CH3COOH contains primilarly CH3COOH molecules, with relatively few H3O+ and CH3COO- ions present

Question 4

What determines if an acid-base reaction goes towards completion or not?

Answer 4

• an acid base reaction involving a strong acid or strong base tends to go to completion (at least one of the reactant is essentially used up)
• if the reaction involves a weak acid and a weak base, the reaction does not go to completion

Question 5

What is the endpoint of a titration?

Answer 5

The point which the colour of the solution changes permanently

Question 6

Equivalence point

Answer 6

When the reactants are in stoichiometric proportion and the reaction is complete and no excess reactant remains

Question 7

Standardization meaning

Answer 7

• in part B of the experiment, you will titrate an unknown acid with a standardized solution (a solution of known concentration) of sodium hydroxide
• to obtain a solution of NaOH of known concentration, you will titrate the NaOH solution against a primary standard acid solution whose concentration is precisely known

Question 8

Primary standard definition

Answer 8

A highly purified solid compound use to determine the concentration of the known solution in a titration
- must be weighed and diluted with great precision because any error will affect the accuracy of the entire analysis

Question 9

What is the primary standard used for titration?

Answer 9

Oxalic acid dihydrate

Question 10

What is phenolphthalein

Answer 10

A complex weak organic acid

Question 11

What is the colour of the proteonated form of phenolphthalein?

Answer 11

Colourless

Question 12

What is the colour of the unprotonated form of phenolphthalein?

Answer 12

Red/pink

Question 13

What is the colour change phenolphthalein is associated with?

Answer 13

When the pH changes from 8 to 10

Question 14

What is a secondary standard?

Answer 14

The solution that is standardized against the primary standard
- in this experiment, the secondary standard is NaOH

Question 15

How do you differentiate between mono-, di-, and tri-protic acids?

Answer 15

The number of hydrogens

Question 16

Preparation of 100 mL of standard oxalic acid

Answer 16

Weigh the calculated amount of solid oxalic acid dihydrate needed to prepare 100 mL of 0.1 M oxalic acid
- weigh solid into 50 mL beaker, transfer the solid to 100 mL beaker

Question 17

Quantitative transfer of oxalic acid

Answer 17

Add 50 mL of DI water to dissolve acid in 100 mL beaker, prepare 100 mL volumetric flask, and then quantitively transfer oxalic acid solution using DI water into 100 mL volumetric flask from the beaker

Question 18

Quantitative transfer meaning

Answer 18

All the material you are investigating must be moved from one place to another without any loss of sample

Question 19

Standardization of NaOH solution

Answer 19

• make sure burette is clean and rinse 3 times with a few millilitres of NaOH solution
• use a volumetric pipette to transfer 10 mL of oxalic acid into a clean 125 mL Erlenmeyer flask
• add 1 drop of phenolphthalein
• from the burette, add NaOH solution to the flask, swirling as you titrate
• repeat 3 times

Question 20

Titration of the unknown acid

Answer 20

• weigh 0.9 g of an unknown acid on the top loading balance using 50 mL beaker
• weigh three samples of the unknown by difference, weighing approximately 0.3 g each into three clean 250 mL Erlenmeyer flasks
• add approx. 100 mL DI water to each sample
• add 2 drops of phenolphthalein to each sample and titrate with standardized NaOH

Question 21

How many significant digits are in the value for the volume of the primary standard solution?

Answer 21

There are 5 significant digits in this volume

Question 22

What volume of oxalic acid standard is used in each titration, with the correct number of significant digits based on the tool used to measure the solution?

Answer 22

Using a volumetric pipette, measure 10.00 mL of the oxalic acid solution

Question 23

At the end point of these titrations, will the volume of sodium hydroxide titrant be the same?

Answer 23

No

Question 24

If the experimenter forgot to record the trial number on the titration flasks, could the results of this titration be used to identify which sample was which?

Answer 24

Maybe, if the sample masses were quite different from each other, the lowest mass could be assumed to use the smallest volume of titrant, etc. and in this way the samples could be identified