Amount of substance

• Describe how to find a volumetric solution

  - weigh the weighing boat containing solid on a 2 d.p balance

  - transfer the solid to a beaker and reweigh the weighing boat

  - record the difference in mass

  - add deionised water and stir with a glass rod until all the solid has dissolved

  - transfer to a volumetric flask with washings

  - make upto 250cm3 mark with distilled water

  - invert/shake the flask
• Why do you need invert/shake the flask?

  - mixes contents and ensures a uniform concentration of solution
• Why must hot solutions not be put in a volumetric flask?


  - the solution would expand and volume wouldn't be correct
• What properties would the solutes need to have before preparing a volumetric solution?

  - not absorb/lose moisture from environment, would change the concentration

  -have a relatively high Mr
• What is a volumetric solution?

  - a solution with a known mass of solute is dissolved in a precise amount of solvent
• Describe how you should weigh out a required mass given a weighing boat

  - weigh the weighing boat to 2dp

  - transfer the solute to beaker

  - reweigh the weighing boat

  - record difference in mass
• Describe how you would make up a volumetric solution from a weighed mass

  - add 100cm3 of deionised water to the beaker

  - stir with a glass rod to dissolve the solid

  - pour solution into a 250cm3 graduated flask via a funnel

  - rinse funnel, beaker and glass rod

  - add washings to the volumetric flask

  - make solution up to graduated mark

  - invert/shake flask several times
• Describe how you would set up equipment for a titration

  - rinse all equipment with distilled water and the solutions that are to be added

  - fill burette with acid

  - add a few drops of indicator

  - release few drops in a waste beaker to remove air bubbles

  - graduated pipette add 25cm3 of alkali
• How can the percentage uncertainty be improved when making a volumetric solution?

  - using a larger mass of solid

  - weight the mass on a balance to more decimal places gives a smaller percentage error
• Describe how you would accurately measure the volume in a burette (2)

  - read the volume from the bottom of the meniscus (1)

  - hold a piece of paper behind the burette to cast a shadow to make it easier to read (2)
• Explain how the use of a burette contributes to the precision of the titre value (2)

  - the precision of a measurement is determined by the limits of the scale on the instrument used (1)

  - a burette has divisions of scale of 0.1cm3 and so precise readings can be recorded with minimal uncertainty (2)
• If distilled water is added from a wash bottle to the conical flask, state the effect, if any, on the titre (3)

  - no effect on titre (1)

  - solution will become more dilute, there is the same number of moles of reagent and so titre will remain the same (2)

  - because the number of moles of each reagent that are needed to cause a colour change in the indicator is the same (3)
• Explain why concordant results are used to calculate the mean titre (2)

  - concordant results are within 0.1cm3 of each other and are more likely to be reliable (1)

  - used to calculate a mean, reduces the human error in titration (2)
• Explain why it is important to rinse out burette/pippette with the reagent it will hold before practical (1)


  - so glassware is coated with correct reagent (1)
• Explain why it's important to rinse out the conical flask with distilled water before it's used in titration (1)

  - pipette is used to transfer a certain number of moles of a solution into the conical flask (1)

  - rinsing the conical flask before hand with anything else will add to the number of moles and lead to an wrong result (2)
• Explain why the air bubbles increases the final burette reading of the rough titration (1)

  - air bubble takes up volume that would otherwise be filled with acid (1)
• State why it is important to fill the space below tap in the burette with solution before beginning an accurate titration (1)


  - the space will be filled during the titration (1)
• Suggest why repeating a titration makes the value of the average titre more reliable

  - remove anomalies

  - calculate a mean
• Why are percentage yields always less than 100%?

  - reaction didn't go to completion - reactions are reversible

  - there might be side reactions - reactants react differently than expected

  - lose some reactant when separating from the reaction mixture eg. When filtering
• The student rinses the burette with distilled water before filling with NaOH, state and explain the effect that the rinsing will have on the value of the titre (2)

  - this would dilute the NaOH that you add (1)

  - so the titre volume would be larger (2)
• Define relative atomic mass

  - the average mass of one atom of an element compared to 1/12 the mass of one atom of carbon 12
• Calculation


  - number of particles = moles x avogadro's constant
• Define empirical formula


  - represents the simplest whole number ratio of the atoms of each element present in a compound
• Define molecular formula


  - gives the actual number of atoms of each element in one molecule of the compound
• State why it is good practice to rinse the pippete with the volume about to be used (1)

  - so titration is done with known concentration of barium hydroxide
• Suggest a reason for the difference between the experimental value for and a correct value

  - didn't heat the solid to a constant mass/didn't heat for long enough
• calculation for finding the number of particles?

  Number of particles = avogadro’s number x number of moles
• what does an acid and alkali form?

  acid + alkali ——— salt + water
• what is the theoretical yield?

  the amount of a product produced assuming no products are lost and all the reactants react fully
• What happens if you add too much indicator when titrating?

  — may react and affect endpoint reading

  — use a smaller volume
• what happens if you rinse the burette with distilled water during titration?

  — slightly dilute the alkali solution

  — instead rinse the burette with the alkali or acid solution