Topic 14 - Quantitative Analysis

Question 1

5.8C - How do you calculate the concentration of a solution using a volumetric flask?

Answer 1

Concentration is measured with a volumetric flask, where its volume is measured by pouring the solution until the graduation mark.
The volume must be noted from eye level for an accurate measure, while doing this the bottom of the meniscus must be touching the graduation mark.

Question 2

5.8C - What is concentration?

Answer 2

The amount of solute dissolved in a certain amount of volume of a solution.
The units of concentration is g/dm3 ( 1dm3 = 1000cm3 )
concentration = mass/volume
The units of concentration can also be moles/dm3
concentration = moles/volume
concentration in mol/dm3 = concentration in g/dm3 / mr of solute.

Question 3

5.9C - Core Practical : Acid-alkali titration using a burette, pipette and suitable indicator

Answer 3

Acid-alkali titrations are used to find the exact volume of alkali needed to neutralise an acid.
Indicators are used as acids and alkalis are colourless.
Rinse a burette with distilled water, then fill the burette with acid, record the initial volume.
Rinse the volumetric pipette, then fill it to the 25cm^3 mark and empty into the flask.
Add a few drops of phenopthalein to the alkali and place a white tile under the flask.
Add acid to the alkali while slowly swirling the flask.
When the indicator changes colour from pink to colourless, drip the acid until the solution is fully colourless.
Record final volume of acid in the burette.
Repeat the experiment until concordant results of 0.1cm^3 between titres.

Question 4

5.10C - How do you calculate the number of moles and concentration in a titration reaction?

Answer 4

Calculate the number of moles of the solution with known volume and concentration.
Use molar ratio to find the moles of the other solution.
Calculate concentration of other solution.

Question 5

5.11C - How do you calculate percentage yield?

Answer 5

The maximum amount of product that can be formed is the theoretical yield.
It is calculated from the balanced equation.
Although during experiments you may not get this much, the amount of product obtained in an experiment is the actual yield.
percentage yield = actual/theoretical yield * 100
Higher the % yield, more useful the reaction as fewer raw materials to make the same amount of product.

Question 6

5.12C - What are some reasons for the actual yield is less than the theoretical yield?

Answer 6

Theoretical yield is calculated assuming that all the products form reactants, this may not be the case because:
the reaction may be incomplete so all the reactants wouldn’t have reacted because the reaction may be not left long enough or could have reached equilibrium.
some of the product could be lost, maybe when the products are transferred between equipment.
there could be unwanted side reactions taking place which could form different products.

Question 7

5.13C - What is the atom economy of a reaction?

Answer 7

Atom economy is a method to show how efficiently a reaction uses its atoms in a reaction. It shows by % the mass of the useful product.
For example ethanol can be made by reacting it with steam, which has 100% yield and also by fermentation, where CO2 is a by-product making the yield of this reaction lower.

Question 8

5.14C - How do you calculate atom economy?

Answer 8

Atom economy = mr of useful product/mr of all the reactants

Question 9

5.15C - How do chemists decide on the best way to manufacture a product?

Answer 9

They have different reaction pathways available to produce the same product.
To figure out which pathway to use, the atom economy is calculated and improved.
The % yield isn’t used here as it doesn’t tell us about the amount of waste product formed, this could mean that more resources need to be put into treating the waste.
To improve the atom economy, chemists find uses for by-products and consider factors such as : energy consumption, rate of reaction, raw materials used and the conditions needed.

Question 10

5.16/17C - What is a molar volume of gas?

Answer 10

it is the volume occupied by one mole of molecules of any gas, that volume is 24dm^3 or 24000cm^3.
To find the moles of gas = volume of gas/molar volume.

Question 11

5.18C - What is Avogadro’s law?

Answer 11

Volume occupied by gases depend on their temperature, pressure and number of molecules.
The Law states that if the temperature and pressure of two gases are the same, then equal volumes of the two gases have equal number of molecules.