Quantitative chemistry (2) - (CCEA)

• The concentration of a solution can be calculated from the number of moles of a substance and the volume of solution it has been dissolved in.

• A titration is practical technique where two solutions are accurately reacted with each other. Titrations be used to calculate unknown concentrations of solutions and to identify unknown compounds.

• Avogadro’s Law states that equal volumes of any gas will contain the same numbers of particles under the same conditions.

• For a chemical process, atom economy is a measure of the amount of atoms in the reactants that end up as useful products. Chemists aim for high atom economies in their reactions.

The equation above can be rearranged to find the number of moles of solute in a solution.

Rearranging the equation: number of moles = concentration (mol/dm³) × volume (dm³)

This equation above can also be rearranged to find the volume of a solution.

Rearranging the equation:

volume (dm³) = moles ÷ concentration (dm³)

Concentrations can also be expressed in grams per decimetre cubed (g/dm³).

Use the relative formula mass (M_r) to convert the two units of concentration.

A titration is an experimental technique that is used to accurately find the volumes of acid and alkali solutions that will react together.

This information can be used in calculations to work out the concentration of an unknown solution.

The equipment below needs to be prepared before use in a titration:

1. A pipette should be rinsed with deionised water and then with the solution which is it is going to be used to measure.

2. A safety pipette filler should be used to draw up the solution until the bottom of the meniscus is on the line and the solution is then released into a conical flask.

3. A burette is rinsed with deionised water and then with the solution which is going to be used in the burette, ensuring the solution passes through the jet.

4. The burette is then filled with the solution, ensuring the jet is filled.

The slideshow below shows the steps involved in titrating an acid into an alkali.

The same steps could be used to titrate an alkali into an acid.

1. Use the pipette and pipette filler to add a measured volume of hydrochloric acid solution to a clean conical flask.
2. Add a few drops of indicator and put the conical flask on a white tile.
3. Fill the burette with sodium hydroxide solution and note the initial reading.
4. Slowly add the alkali from the burette to the acid in the conical flask, swirling to mix. Add the alkali drop by drop toward the end for maximum accuracy.
5. Stop adding the alkali when the indicator first permanently changes colour (the end point). Note the final reading.
6. Repeat steps 1 – 5, until you get concordant (agreeing) titres.

Two common indicators used in titrations are methyl orange and phenolphthalein:

To ensure accuracy in a titration:

• Read volumes from the bottom of the meniscus.
• Add the solution from the burette drop-by-drop near the end point.
• Swirl the conical flask during the titration to ensure the solutions are mixed.

Record the results in a table.

Record your readings to one decimal place.

For example, a reading of 0 cm³ or 24 cm³ on the burette must be recorded as 0.0 cm³ and 24.0 cm³.

The titre is the volume added (the difference between the end and start readings).

A rough titration is carried out to get an idea of the end point of the titration so the accurate titration may be carried out more quickly.

The solution in the burette should be added dropwise, whilst swirling the conical flask, as you approach the end point.

Concordant titres are within 0.2 cm³ of each other.

The results of titrations 1 and 2 are concordant titres.

Please use the link below to access the article on: Prescribed practical (C8) - Titration

(https://truemac.vn/bitesize/articles/zvr3239)

The results of a titration can be used to calculate the concentration of a solution, the volume of a solution, or to identify an unknown compound.

Worked example

In a titration, 25.0 cm³ of 0.1 mol/dm³ sodium hydroxide solution are exactly neutralised by 20.0 cm³ of a dilute solution of hydrochloric acid.

Calculate the concentration of the hydrochloric acid solution in mol/dm³.

In this type of calculation you need to work out the volume of one solution needed to exactly neutralise a certain volume of another solution.

Worked example

25.0 cm³ of 0.3 mol/dm³ sodium hydroxide solution is exactly neutralised by 0.1 mol/dm³ sulfuric acid.

Calculate the volume of sulfuric acid required.

Two types of unknown compounds can be identified using a titration:

• A compound with an unknown element – e.g. MOH (where M is an unknown metal).
• A hydrated compound with an unknown degree of hydration – e.g. Na₂CO₃·xH₂O (where x is the unknown degree of hydration).

In both cases the compound can be fully identified by calculating its M_r.

Worked example

2.8 g of an unknown metal hydroxide, MOH, was dissolved in water to give 25.0 cm³ of solution.

The solution was reacted with 2 mol/dm³ hydrochloric acid in a titration.

25.0 cm³ of the hydrochloric acid was required for neutralisation.

Identify the unknown metal hydroxide.

Neutralisation equation: MOH + HCl → MCl + H₂O

A_r of O = 16, H = 1.

Avogadro's law states:

• equal volumes of different gases contain an equal number of particles under the same conditions of temperature and pressure.

This means that equal numbers of moles of gases occupy the same volume under the same conditions of temperature and pressure.

For example:

• 1 mol of hydrogen gas occupies the same volume as 1 mol of methane gas.

• 10 mol of nitrogen monoxide gas occupy the same volume as 10 mol of argon gas.

Key fact

The volume of 1 mol of any gas at room temperature and pressure is 24 dm³/24000 cm³.

Equal numbers of moles of any gas occupy the same volume.

In reactions where all of the reactants and products are gases, the mole ratio can be used to calculate the volumes of gases.

For example:

Hydrogen reacts with chlorine to form hydrogen chloride:

H₂(g) + Cl₂(g) → 2HCl(g)

The mole ratio of H₂ : Cl₂ : HCl is 1 : 1 : 2.

Therefore:

• 1 cm³ of hydrogen gas would react with 1 cm³ of chlorine gas to produce 2 cm³ of hydrogen chloride gas.

• 20 dm³ of hydrogen would react with 20 dm³ of chlorine gas to produce 40 dm³ of hydrogen chloride gas.

Some products in chemical reactions are desired, and others are not desired (sometimes called by-products).

For example, in the manufacture of hydrogen from methane and steam:

methane + steam → hydrogen + carbon monoxide

The hydrogen is the desired product and the carbon monoxide is the by-product (not-desired).

In a chemical reaction atoms in the reactants rearrange themselves to form the products.

Atom economy is a measure of the amount of the reactant atoms that end up as useful products.