It is important to know the correct quantities of reactants to use in a synthesis, and how much product has been produced. It is also important to know how pure the product is - this can be tested using the method of titration.
HOW TO CARRY OUT TIRATION?
Calculating relative formula masses
Relative atomic mass
The relative atomic mass of an element shows its mass compared with the mass of atoms of other elements. The relative atomic mass of carbon is 12, while the relative atomic mass of magnesium is 24. This means that each magnesium atom is twice the mass of a carbon atom.The relative atomic mass of each element can be found in the Periodic Table.
Relative formula mass
Relative atomic masses can be used to find the relative formula mass of a compound.Here are two examples:
- Question
- Find the relative formula mass of sulfuric acid H2SO4.
- Answer>>>
- Sulfuric acid has the formula H2SO4. The relative atomic masses of the elements in sulfuric acid are H = 1, O = 16 and S = 32. Adding together the relative atomic masses of each atom in the formula of sulfuric acid gives the relative formula mass of sulfuric acid: H2SO4 relative formula mass = (2 x 1) + 32 + (4 x 16) = 98
- Question
- Find the relative formula mass of sodium hydroxide NaOH.
- Answer>>>
- Sodium hydroxide has the formula NaOH. The relative atomic masses of the elements in sodium hydroxide are H = 1, Na = 23 and O = 16. Adding together the relative atomic masses of each atom in the formula of sodium hydroxide gives the relative formula mass: NaOH relative formula mass = 23 + 16 + 1 = 40
Percentage yield
The yield from a chemical reaction is the mass of product made. A balanced symbol equation shows the number of atoms and molecules in the reactants and products. The reactants are on the left of the arrow and the products are on the right of the arrow. A balanced equation can tell us the theoretical yield for a reaction, by using the relative formula masses of reactants and products involved. For example the formula for the neutralisation of sulfuric acid with sodium hydroxide is: sulfuric acid + sodium hydroxide → sodium sulfate + water H2SO4 + 2NaOH → Na2SO4 + 2H2O By using the relative formula masses it can be calculated that 6.9g of sulfuric acid theoretically produces 10g of sodium sulfate. If this neutralisation reaction is carried out, the actual yield is found by measuring the mass of product - sodium sulfate - that has been made. The actual yield of sodium sulfate produced will be less than the theoretical yield calculated using the equation.
Theoretical vs actual yield
The difference between the theoretical yield and the actual yield for any reaction may have one or more of a number of causes:
- Some of the reactants may remain unreacted when the reaction is complete.
- Some of the product may be lost when liquids or solids are transferred from one container to another.
- Some of the reactants may form other products.
percentage yield = (actual yield ÷ theoretical yield) × 100- Question
- In the neutralisation of sulfuric acid with sodium hydroxide, the theoretical yield from 6.9g of sulfuric acid is 10g. In a synthesis, the actual yield is 7.2g. What is the percentage yield for this synthesis?
Answer>>>
- percentage yield = (actual yield ÷ theoretical yield) × 100 percentage yield = (7.2 ÷ 10) x 100 = 72%
Carrying out a titration
Measuring purity
For some synthesised chemical compounds - pharmaceuticals, for example - it is very important that they have a high level of purity. Just a tiny amount of an impurity in a drug could cause a great deal of harm to a patient. Samples of chemicals that are synthesised must be checked for purity. This is often done by carrying out a titration. A titration is used to measure the volume of one solution that exactly reacts with another solution.
Steps of titration
A titration is carried out using a number of steps:
- If the sample is a solid, it is weighed using an accurate balance, and then dissolved to make up a known volume of solution (usually 100cm3).
- A pipette is used to measure accurately a volume of this solution - for example, 10cm3. A safety pipette filler is used to draw solution into the pipette. This is emptied into a conical flask.
- A few drops of an indicator may be added to the conical flask. This will show a change of colour when the titration is complete.
- A second chemical is placed in a burette. This other solution is of a chemical that will react with the synthesised chemical sample in the conical flask. Often the solution in the burette is an acid or alkali, and it must be of a precise, known concentration.
- The solution from the burette is run into the conical flask. The solution is added one drop at a time, with swirling to mix the solutions as the end-point is approached. Eventually, a colour change shows that the correct amount has been added to react completely with the synthesised chemical in the sample.
- The volume of solution added from the burette is noted. The titration results can then be used to calculate the amount of the synthesised chemical in the sample, and therefore find its purity.
Apparatus required for titration (from left to right): burette, conical flask, safety pipette filler and pipette
Calculations from equations - Higher
A balanced symbol equation can be used as the starting point to calculate the quantities of reactants used, and the quantities of products made, in a reaction.
Step 1: write down the balanced symbol equation
Sulfuric acid is neutralised by sodium hydroxide, to make sodium sulfate and water, according to the following equation:
H2SO4 + 2NaOH → Na2SO4 + 2H2O
Step 2: work out the relative formula mass
The relative formula masses of the reactants and products can be worked out using relative atomic masses obtained from the periodic table.
H2SO4 relative formula mass = (2 x 1) + 32 + (4 x 16) = 98
NaOH relative formula mass = 23 + 16 + 1 = 40
Na2SO4 relative formula mass = (2 x 23) + 32 + (4 x 16) = 142
H2O relative formula mass = (2 x 1) + 16 = 18
Step 3: work out the masses of reactants and products
The masses of reactants and products can now be worked out from the equation:
H2SO4 + 2NaOH → Na2SO4 + 2H2O
98 + (2x40) = 142 + (2x18)
98 + 80 = 142 + 36
These numbers can now be applied to any unit of mass - grams, kilograms, tonnes, and so on. Now try the questions below.
- Question......
- What mass of sulfuric acid reacts with sodium hydroxide to produce 10g of sodium sulfate?
- Question.......
- What mass of sodium sulfate is made when 5.0 tonnes of sodium hydroxide is reacted with sulfuric acid?