The standard calculation
5 marks · HardA chemistry student carries out a titration to determine the concentration of a potassium hydroxide () solution. She finds that 25.0 cm³ of solution of unknown concentration reacts exactly with 20.0 cm³ of 0.200 mol/dm³ sulfuric acid (). The balanced equation for the reaction is:
Use the three-step titration calculation method to find the concentration of the solution.
Calculate the number of moles of reacted.
Use the balanced equation to work out the number of moles of that reacted.
Calculate the concentration of in mol/dm³.
State the three-step titration calculation method used in this problem.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) 0.00400 mol
(b) 0.00800 mol
(c) 0.320 mol/dm³
(d) 1) Calculate moles of known solution; 2) Use balanced equation for moles of unknown; 3) Calculate concentration using volume and moles.
Common mistakes: Incorrect volume conversion; Incorrect stoichiometric ratio; Not dividing by the correct volume when calculating concentration
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Marking points
M1 (a) · 1 mark · Correct calculation of moles of :
M2 (b) · 1 mark · Correct use of mole ratio (1 mol to 2 mol ) to calculate moles of : (follow-through)
M3 (c) · 1 mark · Correctly converts volume of to dm³: (follow-through)
A1 (c) · 1 mark · Accurately calculates concentration: (follow-through)
B1 (d) · 1 mark · States the three-step titration calculation method: 1) Calculate moles of known; 2) Use balanced equation; 3) Calculate concentration.
Worked solution
(a) Number of moles of :
(b) From the equation, 1 mol reacts with 2 mol , so:
(c) Concentration of :
(d) Three steps: 1) Calculate moles of known solution; 2) Use the balanced equation (mole ratio) to find moles of unknown; 3) Calculate concentration using moles and volume.