Concentrations of Solutions

A description of how to carry out concentration calculations based on titration results.

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  • Created by: R_Hall
  • Created on: 24-02-13 12:39

Concentration

  • Concentration is how much of a substance is dissolved up in a known volume
  • When we make a solution, the concentration depends on the amount (or volume) of solute and the final volume of the solution
  • Concentration is always for moldm-3, sometimes abbreviated to M
  • 1000 cm cubed= 1 dm cubed
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Steps

  • 1. Write the balanced symbol equation for the reaction (use state symbols)
  • 2. Find the ratio of reactants
  • 3. Calculate the number of moles of the acid used to neutralise the alkali
  • 4. Use the ratio from step 2 to work out the number of moles of alkali used
  • 5. Calculate the concentration of the alkali using the answer from step 4 and the concentration triangle to help you
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Worked example

  • In a titration, 25cm3 of potassium hydroxide was pipetted into a conical flask. A 0.02 moldm-3 solution of sulphuric acid was added from a burette. Phenolphtalein indicator was used which changed colour from pink to colourless when 19.00 cm3 of sulphuric acids was added. What was the concentration of the potassium hydroxide?
  • 1. Equation- 2KOH + H2SO4 -> 2H2O + K2SO4
  • 2. Ratio- 2:1
  • 3. Moles of acid- n=cv, 0.02x 0.019 (19/1000)= 3.8 x 10-4 moles
  • 4. Moles of alkali- 3.8 x 10-4 x 2= 7.6 x 10-4 moles
  • 5. Concentration alkali- c= n/v, 7.6 x 10-4 / 0.025 (25/1000)= 0.03 moldm-3
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Percentage errors

  • For graduated equipment the uncertainty/ precision error is taken to be half a division on either side of the smallest unit on the scale you are using
  • 250 cm3 volumetric flask- uncertainty = +/- 0.2 cm3. percentage uncertainty= 0.2/250 x 100= 0.08%
  • 25 cm3 pipette- uncertainty= +/- 0.06 cm3. percentage uncertainty= 0.06/25 x 100= 0.2%
  • Burette- uncertainty= +/- 0.05 cm3. difference between 2 readings = 0.05 x 2= +/- 0.1 cm3
  • 2 dp balance- uncertainty= +/- 0.005 g. difference between readings= 0.005 x 2= +/- 0.01 g
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