SC14 - Quantitative Analysis ✓

Question 1

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

Answer 1

• Theoretical yield: The maximum mass of product that can be formed from the reactant
• 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

Percentage Yield = Actual Yield / Theoretical Yield

X 100

Question 3

SC14a - What are the 3 reasons that actual yield may be less than theoreticla yield?

Answer 3

1. the reaction may be incomplete, so not all of the reactants are used up - possibly because the reaction has not been left for long enough or the reaction may reach equilibrium
2. some of the product is lost - for example, when a liquid is transferred from one container to another
3. there may be other unwanted side reactions taking place - for example some reacts may react to make a different product

Question 4

SC14a - Why is a higher percentage yield better?

Answer 4

• A high yield means that fewer raw materials are used to make the same amount of product, so there is less waste and more profit can be made

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) ÷ (Mr or Ar)

Question 8

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 8

• 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 colourless; 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 9

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

Answer 9

24dm³

24 000 cm³

Question 10

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

Answer 10

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