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In synthesizing benzoic acid using 3mL benzyl alcohol, HCl, Na2CO3 and KMnO4.....?
1) During filtration the hot solution, we got colourless solution. But, when we place it in ice bath, its colour turn to light pink. Does it mean there are impurities? How to avoid it?
2) The colourless solution result from filtration has been added concentrated HCl and stir until the solution become acidic. Then, my lecturer ask to add HCl more and more again. Why? What is the effect of HCl in this case?
3)After drying the crystal, how to calculate percentage yield? How to get the theoretical yield? If it is from the balanced equation, can u teach me how? Could the actual yield greater than theoretical yield?
1) Mn(II) you get in this reaction is slightly pink
2) you add HCl to protonate benzoic acid - it's less soluble in water than sodium benzoate and you can purify it through crystallization
3) you calculate it based on the equation - you need to convert the volume of the alcohol into grams - m=d*V (where m is mass in grams, d is density and V is volume, 3 ml)
then:
C6H5CH2OH +2 [O] -> C6H5COOH +H2O (even if you write the whole equation with KMnO4 as the oxidant, you'll still have 1:1 molar ratio of benzyl alcohol:benzoic acid)
MW of benzyl alcohol is 108.14 g/mol and that of benzoic acid is 122.12 g/mol
so theoretical yield will be y=122.12*m/108.14 (m - is the mass, I found it to be 3.13 with d=1.044 g/ml). I calculated it to be 3.54 g
actual yield is the amount you get from the reation divided by 3.54 and then multiplied by 100%. It's always lower than the theoretical yield unless: i) you added more benzyl alcohol than 3 ml (ii) you have impurities
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