✔ 最佳答案
What is back titration ?
Back titration is an indirect method of titration. In a back titration, the substance being quantitatively determined is completely reacted with an excess of known amount reagent. The quantity of this unreacted reagent is then determined by using a simple titration.
For example, consider the titration in the question. The quantity of magnesium oxide is determined. Magnesium oxide is firstly completely reacted with an excess of known amount of hydrochloric acid. The quantity of unreacted hydrochloric acid is determined by using a simple titration with standard sodium hydroxide solution. The quantity of magnesium oxide is then determined indirectly using the data obtained.
A back titration is used when the substance cannot be quantitatively determined by using simple titration. In this case, magnesium oxide is an insoluble solution, and it is impossible to titrate a solid using a simple titration. Therefore, a back titration is used.
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Calculations :
Firstly, consider the reaction of the excess HCl with NaOH solution :
HCl + NaOH → NaCl + H2O
Mole ratio excess HCl : NaOH = 1 : 1
No. of moles of NaOH used = MV = 0.2 x (19.7/1000) = 0.00394 mol
No. of moles of excess HCl = 0.00394 mol
Then, consider the reaction of MgO with HCl :
MgO + 2HCl → MgCl2 + H2O
Mole ratio MgO : HCl = 1 : 2
Total no. of moles of HCl = MV = 2 x (100/1000) = 0.2 mol
No. of moles of HCl reacted = 0.2 - 0.00394 = 0.19606 mol
No. of moles of MgO used = 0.196/2 = 0.09803 mol
Molar mass of MgO = 24.3 + 16 = 40.3 g mol-1
Mass of MgO = mol x (molar mass) = 0.09803 x 40.3 = 3.95 g
% by mass of MgO = (3.95/4.06) x 100% = 97.3%
