SC14a - What is the difference between actual and theoretical yield?
- Theoretical yield: The yield you would expect to get based off the ratio of moles
- Actual yield: The yield you get once the experiment has been conducted
SC14a - How do you calculate percentage yield?
Percentage yield compares actual yield to theoretical yield
PY = (AY ÷ TY) x 100
SC14a - What are the reasons that actual yield may be less than theoreticla yield?
- The reaction has not been left for long enough/has reached equilibrium
- Practical losses such as liquids being left in containers and gases escaping
- Unwanted side reactions taking palce can use up some reactants to make a different product
SC14a - Why is a higher percentage yield better?
It means there has been a more efficient use of the products
SC14b - How do you work out atom economy and what does it show you?
- It shows you the perentage of atoms that have gone into making useful products and how many atoms you are wasting
- Allows you to compare and work out the most efficient way fo making a product
- Atom economy = (Mr of useful product(s) ÷ Sum of Mr of reactants) x 100
- Mr can also be Ar
SC14b - What is one way of improving atom economy?
Finding uses for the by-products of the reaction
SC14c - How do you convert concentrations from mol/dm³ and g/dm³
Using the molar mass forumla
mol/dm³ = (g/dm³) ÷ (m/dm³)
SC14d - How do you work out the volume or concentration of an alkali needed to neutralise a given acid?
- Firstly work out the balanced equation (which may already be given) and work out the molar ratio of the acid and alkali reactants
- In a 1:1 ratio, the concentrations will be inversely proportionate to the volumes used (will multiply to make the same number)
- If not then multiply the concentration and volume of the acid.
- Divide it by its side of the ratio and multiply it by the other
- This will give you the moles involved
- Divide this by either the concentration or volume of the alkali to find the other
SC14d CP - Describe the method used to carry out an acid-alkali titration with hydrochloric acid (HCl) and 25cm³ of Sodium Hydroxide solution (NaOH)
- Use a pipette to measure out 25cm³ of sodium hydroxide and empty this solution into a conical flask
- Place the conical flask on a white tile (So you can see the colour change later on)
- Wash out a burette with hydrochloric acid and fill it up to top with this.
- Record the inital reading on the burette (from the bottom on the meniscus)
- Add a few drops of indicator (methyl orange or phenolphthalein) to the conical flask and move this and the white tile under the burette
- Open the tap of the buretted letting the acid flow through. Constantly swirl the flask
- When you first see a colour change, slow down the tap
- For phenolphthalein this will be pink to clourless; For methyl orange this will be yellow to red
- Once the colour has changed and won't change back, stop the tap and read the value on the burette (from the bottom of the meniscus)
- Work out how much acid has been used
- Repeat multiple times and take an average of the concordant results
SC14e - What is the Molar gas volume at rtp (room temperature and pressure)
24 000 cm³
SC14e - What is the formula for the moles in a gas?
Moles = Volume ÷ Molar gas volume (24dm³ at rtp)