One method for determining the purity of aspiring (C9H8O4) is to hydrolyze it wi

Question

One method for determining the purity of aspiring (C9H8O4) is to hydrolyze it with NaOH solution and then to titrate the remaining NaOH. The reaction of aspiring with NaOH is as follows: C9H8O4(s) + 2OH^-(aq) --> C7H5O3^-(aq) + C2H3O2^- (aq) + H2O(l) A sample of aspiring with a mass of 1.327 g was boiled in 50.00ml of 0.500 M NaOH. After the solution was cooled, it took 31.92 ml of 0.289 M HCl to titrate the excess NaOH. Calculate the purity fo the aspiring. What indicator should be used for this titration? Why?

Explanation / Answer

moles of OH- = moles of NaOH = 50x0.5/1000 = 0.025, moles of compound = 1.327/ (180) = 0.00737 , moles of OH- reacted = ( 0.00737 x2) = 0.01474 , OH- left = ( 0.025-0.01474) = 0.01026 ( theoretically left OH-) , moles of acid H+ = 31.92 x0.289/1000 = 0.009225 = OH-( observed OH- left) now ( 0.01026-0.009225) = 0.001035 is moles of OH- is extra calculated , hence compound moles extra calculated = ( 0.001035/2) = 0.00051756 , hence compound real moles = ( 0.00737 -0.00051756) = 0.00685 , mass of compound reacted = 0.00685 x180 = 1.2334 gm , purity = ( 1.2334/1.327) x100 = 92.95 % pure , since acid is weak and base is strong phenophthelin can be used as indicator, since at equivalence point which is from pH= 8-10 , phenothelin indicator works i.e shows colour.

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