Chapter 13 - Acids and Bases Analysis

Stoichiometry

The calculation of the relative quantities of reactants and products in chemical reactions

U/K formula

Excess Reactant (Reagent)

The substance which will not be used up completely in the reaction

Limiting Reactant

The substance which will be used up completely in the reaction. The limiting reactant will determine the amount (moles) of products that can be formed.

Reagent

A substance or compound added to a system to cause a chemical reaction, or to test if a reaction occurs

Stoichiometry Summary Steps

1. Balance the chemical equation

2. Calculate amount (number of moles) for reactants

3. Identify the limiting reagent using the U/K formula.

4. Use the U/K formula with the limiting reagent to determine the amount (moles) of the unknown chemical.

5. Convert the amount (moles) of the unknown to the desired units

6. Check for units and significant figures

Volumetric Analysis Aim

To find the unknown concentration (Quantitative technique)

Volumetric Analysis

A quantitative laboratory method used to find the unknown concentration or amount of a substance in a solution by measuring the volume of a standard reacting solution

Titration

A laboratory method used to find the concentration of an unknown solution by reacting it with a solution of known concentration

Titration Steps

1. Pipette a set volume (aliquot) of a solution (known OR unknown) into a conical flask.

2. Add few drops of a suitable pH indicator to the conical flask.

3. Fill the burette with the other solution (unknown if the known in the conical flask and vice versa). Record the initial volume of the burette.

4. Slowly swirl the solution from the burette into the conical flask.

5. Stop adding the solution when there is a permanent colour change (end point) as this indicates the equivalence point.

6. Calculate the volume added (titre) added by subtracting the final volume from the initial volume of the burette.

7. Repeat the titration until three concordant titres (all within 0.10 mL) are obtained.

Volumetric Flask Use

to prepare standard solutions

Volumetric Flask Rinse

Distilled water only

Volumetric Pipette

Calibrated glass tube which delivers a fixed volume accurately. Rinse with first with distilled water and then with the solution going through it.

Aliquot

The volume delivered by a pipette

Conical Flask

Container for aliquot that sits underneath burette during titration and is where chemical reaction occurs. Rinse with distilled water only.

Burette

A burette is a calibrated glass tube that delivers a variable volume accurately. Rinse first with distilled water and then with the solution going through it.

Titre

The volume delivered by a burette

Only rinsing burette with water

Titre is diluted and so a more titre is used to reach the endpoint. Thus, the unknown appears less concentrated and the known appears more concentrated

Only rinsing pipette with water

Aliquot is diluted and so less titre is used to reach the endpoint. Thus, the known appears less concentrated and the unknown appears more concentrated

Primary Standard Criteria

• Obtainable in a pure form.

• Have a known chemical formula.

• High molar mass to minimise the effect of errors in weighing.

• Be easily stored without reacting with water or gases of the atmosphere (i.e., stable).

• Preferably inexpensive.

Primary Standard

An extremely pure, stable, and reliable substance or reference measurement that directly provides a known concentration or value without needing calibration against another standard

Equivalence Point

When the known (standard solution/titrant) reactant and unknown (analyte/titrand) reactant completely neutralise each other.

Reading a Burette

• The volume of solution in a burette is read from top to bottom

• Estimate the titre to 2 decimal places

• Read values from the bottom of the meniscus

• Eye level with meniscus to avoid parallax errors

Concordant Titres

Titration volume measurements that are extremely close to one another, within 0.10 mL of each other

Types of Errors

• Systematic errors

• Random errors

There are exceptions like some human mistakes however.

Systematic Errors

Alter all measurements by a consistent magnitude, typically in one direction.

Random Errors

Alter all measurements by a random magnitude

Dilution Factor

Ratio of the volume in the initial concentrated solution to the volume of the final diluted solution