Section 8.4 – pg. 328-332

Experimental designs discussed so far have been QUALitative (flame test, solution colour, litmus test, conductivity, solubility)

QUANtitative experimental designs allow measures amounts to be obtained.

The experimental design chosen for analysis depends on equipment, time available, and degree of accuracy required.

There are four types of analytical experimental designs:

1. Crystallization – the solvent is vaporized from a solution, with or without heating, leaving a solid behind which is measured for mass.

2. Filtration – a low solubility solid, produced from a single or double replacement reaction, is separated with a filter then dried so mass can be measured (filtrate = solution that goes through)

3. Gas Collection – a gas, formed as a product of a reaction, is collected and volume, pressure and temperature are measured.

4. Titration – a solution in a burette (called the “titrant”) is progressively added to a measured volume of another solution (called the “sample”) in an Erlenmeyer flask. The volume of the titrant at the endpoint is measured.

Titration – is a common experimental design used to determine the amount concentration of substances in solution.

The solution of known concentration may be either the titrant or the sample; it makes no difference to the analysis

Titration breakdown: Carefully adding a solution (titrant)

from a burette into a measured, fixed volume of another solution (sample) in an Erlenmeyer flask until the reaction is judged to be complete

Burette – precisely marked glass cylinder with a stopcock at one end. Allows precise, accurate measurement and control of the volume of reacting solution.

When doing a titration, there will be a point at which the reaction is complete; when chemically equivalent amounts of reactants have combined. This is called the equivalence point:

Equivalence point – the point during a titration at which the exact theoretical chemical amount of titrant has been added to the sample. (QUANTITATIVE)

To measure this equivalence point experimentally, we look for a sudden change in an observable property, such as color, pH, or conductivity. This is called the endpoint. (QUALITATIVE)

An initial reading of the burette is made before any titrant is added to the sample.

Then the titrant is added until the reaction is complete; when a final drop of titrant permanently changes the colour of the sample.

The final burette reading is then taken.

The difference between the readings is the volume of titrant added.

Near the endpoint, continuous gentle swirling of the solution is important

A titration should involve several trials, to improve reliability of the answer.

A typical requirement is to repeat titrations until three trials result in volumes within a range of 0.2mL.

These three results are then averaged before carrying out the solution stoichiometry calculation; disregard any trial volumes that do not fall in the range.

Any property of a solution such as colour, conductivity, or pH that changes abruptly can be used as an endpoint. However, some changes may not be very sharp or may be difficult to measure accurately.

This may introduce error into the experiment. Any difference between the titrant volumes at the empirical (observed) endpoint and the theoretical equivalence point is known as the titration error.

How to read a burette:

**Initial readings will always be lower (1 mL) than final readings (13.2 mL) because 0 is at the top of the burette. You will see this in the example problem next.

Determine the concentration of hydrochloric acid in a commercial solution.

A 1.59g mass of sodium carbonate, Na2CO3(s), was dissolved to make 100.0mL of solution. Samples (10.00mL) of this standard solution were then taken and titrated with the hydrochloric acid solution.

The titration evidence collected is below. Methyl orange indicator was used.

Trial 1 2 3 4

Final burette reading (mL) 13.3 26.0 38.8 13.4

Initial burette reading (mL) 0.2 13.3 26.0 0.6

Volume of HCl(aq) added 13.1 12.7 12.8 12.8

Indicator colour Red Orange

Orange

Orange

TIP: In titration analysis, the first trial is typically done very quickly. It is just for practice, to learn what the endpoint looks like and to learn the approximate volume of titrant needed to get to the endpoint. Greater care is taken with subsequent trials

First: Determine the concentration of the 100.0 mL sodium carbonate solution.

1.59 g Na2CO3(s) x 1 mol x 1 = 0.150 mol/L 105.99g 0.100L

Second: Write a balanced chemical equation:

2HCl(aq) + Na2CO3(aq) H2CO3(aq) + 2NaCl(aq)

*12.8 mL 10.0 mL

c = ? c = 0.150 mol/L

c HCl (aq): 10.0 mL x 0.150 mol x 2 x 1 = 0.234 mol/L L 1 12.8mL

Using titration analysis, the concentration of the commercial hydrochloric acid solution is 0.234 mol/L

* The volume of HCl(aq) used is an average of

trials 2, 3, and 4.

Titration is the technique of carefully controlling the addition of a volume of solution (the titrant) from a burette into a measured fixed volume of a sample solution until the reaction is complete.

The concentration of one reactant must be accurately known.

The equivalence point is the point at which the exact theoretical (stoichiometric) reacting amount of titrant has been added to the sample.

The endpoint is the point during the titration at which the sudden change of an observable property indicates that the reaction is complete.

Several trials must be completed. When at least three trials result in values that are all within a range of 0.2 mL, those values are averaged. The average value is used for the stoichiometry calculation.

Pg. 331 #1-4 – go through answers as a class

Preparing and Reacting Solutions Worksheet – Guided Practice

Volumetric Stoichiometry – Titration Worksheet - homework