Experiment 3: Titration As An Analysis Tool - The Identification Of An Unknown Solid Acid Flashcards
The Bronsted and Lowry Acid and Base Theory
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
The acid strength of a Bronsted-Lowry acid:
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
Weak Acids vs Strong Acids
- 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
What determines if an acid-base reaction goes towards completion or not?
- 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
What is the endpoint of a titration?
The point which the colour of the solution changes permanently
Equivalence point
When the reactants are in stoichiometric proportion and the reaction is complete and no excess reactant remains
Standardization meaning
- 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
Primary standard definition
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
What is the primary standard used for titration?
Oxalic acid dihydrate
What is phenolphthalein
A complex weak organic acid
What is the colour of the proteonated form of phenolphthalein?
Colourless
What is the colour of the unprotonated form of phenolphthalein?
Red/pink
What is the colour change phenolphthalein is associated with?
When the pH changes from 8 to 10
What is a secondary standard?
The solution that is standardized against the primary standard
- in this experiment, the secondary standard is NaOH
How do you differentiate between mono-, di-, and tri-protic acids?
The number of hydrogens
Preparation of 100 mL of standard oxalic acid
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
Quantitative transfer of oxalic acid
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
Quantitative transfer meaning
All the material you are investigating must be moved from one place to another without any loss of sample
Standardization of NaOH solution
- 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
Titration of the unknown acid
- 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
How many significant digits are in the value for the volume of the primary standard solution?
There are 5 significant digits in this volume
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?
Using a volumetric pipette, measure 10.00 mL of the oxalic acid solution
At the end point of these titrations, will the volume of sodium hydroxide titrant be the same?
No
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?
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
