RP1: Volumetric solution and acid–base titration.

• What is a standard solution?


  solution that you know the exact concentration of
• How to make a standard solution


  Dissolve a known amount of solid in a known amount of water to create a known concentration.
• Methyl orange indicator colours


  red in acids and yellow in alkalis
• Phenolphthalein indicator colours


  Colourless in acids and pink in alkalis
• What do titrations allow you to find out?


  The concentration of an alkali or acid
• What are concordant results?


  Titres within 0.1 cm3 of each other
• To convert concentration measured in mol dm-3 into concentration measured in dm-3 multiply by Mr of the substance
• How to make a standard solution for titrations?


  Use moles= conc x vol
• How to make a standard solution for titrations?

  -Use moles= conc x vol

  -Work out mass using Mr

  -Put weighing boat on balance. Weigh mass of solid into a beaker.

  -Weigh weighing boat. Find difference

  -Add distilled water to beaker and stir with glass rod until the solid dissolves.

  -Tip solution into 250 cm3 volumetric flask — use funnel

  -Rinse beaker, stirring rod and funnel with distilled water/washings and add to flask - so no solute left

  -Top the flask using more distilled water.

  -Stopper the flask and invert/shake

  -Calculate the concentration
• How to do a titration?


  -Do a rough titration for the endpoint.

  -Take an initial reading of how much acid is in the burette.

  -Then add the acid to the alkali, and swirl flask.

  -When the colour changes, take a final reading.

  -Then do an accurate titration.

  -Run the acid in to within 2 cm3 of the endpoint and add dropwise.

  -Find the amount of acid used to neutralise the alkali by subtracting the final reading from the initial reading. This is called the titre.

  -Repeat the titration a few times, until you have at least three concordant results.

  -Calculate the mean volume of acid used.
• How to work out total percentage error?


  uncertainty on apparatus x no. of measurements/ value of measurements x100
• What is aliquot?


  A small sample of an acid
• What is a titration?


  A procedure for calculating the concentration of an unknown solution
• Common titration mistakes

  Not filling jet space

  Leaving funnel in burette
• What hazard is hydrochloric acid?


  Corrosive
• A student rinsed the apparatus before starting an acid-base titration. The titration results showed that the volume of acid added from the burette was larger than expected. Which is a possible reason for this?


  The burette was rinsed only with water.
• What would reduce the overall measurement uncertainty in the titration?


  Titre volume needs to be larger.

  Use less concentrated alkali in the burette

  Increase volume and concentration of the substance in the conical flask
• 25.0 cm3 samples of NaOH solution were taken by pipette from a beaker. These were then titrated with an aqueous solution of ethanoic acid. The concentration of ethanoic acid calculated from the experimental results was found to be lower than the actual value. Which of these could explain the difference?


  Rinsing the burette with distilled water before filling with ethanoic acid
• How to reduce uncertainties in mass for titrations?

  Larger mass or weigh mass before and after or more accurate scale
• In a titration what do you rinse the burette with?


  The solution in the burette
• In a titration what do you rinse the pipette with?


  The unknown concentration solution
• In a titration what do you rinse the conical flask with?


  Distilled water
• Where do you read the titration value from?


  Bottom of the meniscus
• Why is a conical flask preferred to a beaker?


  Easier to swirl mixture without spilling
• Why should you use a white tile in a titration?


  To observe colour change
• What happens if you do not fill jet space?


  Larger titre value
• What happens to the concentration if you rinse the burette with water?


  Higher concentration calculated
• What happens to the concentration if you rinse the conical flask with water?


  No effect
• What happens to the concentration if an air bubble is in the pipette?


  Lower concentration calculated
• The student identified use of the burette as the largest source of uncertainty in the experiment.Suggest how to reduce the percentage uncertainty in using the burette.

  Justify your suggested improvement.

  Use a larger mass or a more dilute HCl solution or more conc alkali so a larger titre value will be needed