SC14 - Quantitative Analysis ✓

Question 1

SC14a - What is the difference between actual and theoretical yield?

Answer 1

• 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

Question 2

SC14a - How do you calculate percentage yield?

Answer 2

Percentage yield compares actual yield to theoretical yield

PY = (AY ÷ TY) x 100

Question 3

SC14a - What are the reasons that actual yield may be less than theoreticla yield?

Answer 3

• 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

Question 4

SC14a - Why is a higher percentage yield better?

Answer 4

It means there has been a more efficient use of the products

Question 5

SC14b - How do you work out atom economy and what does it show you?

Answer 5

• 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 = (M_r of useful product(s) ÷ Sum of M_r of reactants) x 100
• M_r can also be A_r

Question 6

SC14b - What is one way of improving atom economy?

Answer 6

Finding uses for the by-products of the reaction

Question 7

SC14c - How do you convert concentrations from mol/dm³ and g/dm³

Answer 7

Using the molar mass forumla

mol/dm³ = (g/dm³) ÷ (m/dm³)

Question 8

SC14d - How do you work out the volume or concentration of an alkali needed to neutralise a given acid?

Answer 8

• 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

Question 9

SC14d CP - Describe the method used to carry out an acid-alkali titration with hydrochloric acid (HCl) and 25cm³ of Sodium Hydroxide solution (NaOH)

Answer 9

• 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

Question 10

SC14e - What is the Molar gas volume at rtp (room temperature and pressure)

Answer 10

24dm³

24 000 cm³

Question 11

SC14e - What is the formula for the moles in a gas?

Answer 11

Moles = Volume ÷ Molar gas volume (24dm³ at rtp)