SC14 - Quantitative Analysis ✓ Flashcards Preview

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Flashcards in SC14 - Quantitative Analysis ✓ Deck (11)
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1

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

2

SC14a - How do you calculate percentage yield?

Percentage yield compares actual yield to theoretical yield

PY = (AY ÷ TY) x 100

3

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

4

SC14a - Why is a higher percentage yield better?

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

5

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

6

SC14b - What is one way of improving atom economy?

Finding uses for the by-products of the reaction

7

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

Using the molar mass forumla

mol/dm³ = (g/dm³) ÷ (m/dm³)

8

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

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)

  • 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

10

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

24dm³

24 000 cm³

11

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

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