Concentrations of Solutions

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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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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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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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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