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Strong and Weak Acids
2018 Paper One
This question is worth 15 marks.
Some students calculated the amounts, in moles, of the reactants but did not multiply the amount of barium hydroxide by two to give the amount of hydroxide ions.
Some students did not convert the amount in moles to concentration
Some did not quote their pH to two decimal places.
Many students failed to recognise the need to increase the amount of acid and decrease the amount of salt due to the shift in the equilibrium when H+ ions are added.
Many added and subtracted the wrong way round or managed one process but not the other.
A strong acid is one that dissociates completely in solution
Moles of hydrochloric acid = 0.100 x 10.35/1000 = 0.001035
Moles of barium hydroxide = 0.150 x 25/1000 = 0.00375 x 2 (dibasic) = 0.0075
Excess OH = 0.0075 – 0.001035 = 0.006465 moles
The concentration of hydrogen ios is equal to the concentration of hydroxide ions
This is the sulfur dioxide
Calculate the moles
moles HCl added = 2 x 0,005 = 0.01 mol
moles CH3COOH = 0.07 x 0.5 = 0.035 mol
moles CH3COO– = 0.025 – 0.01 (= 0.015)
moles CH3COOH = 0.035 + 0.01 (= 0.045)
Volume = (500 + 5)/1000 = 0.505 dm3
Conc CH3COOH = 0.045/0.505 = 0.0891
Conc CH3COO– = 0.015/0.505 = 0.0297
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