Aluminum hydroxide or gibbsite, Al(OH)?, is extensively used as a fire retardant

Question

Aluminum hydroxide or gibbsite, Al(OH)?, is extensively used as a fire retardant. A sample of 0.3250 g of gibbsite-containing rock was analyzed in order to determine its Al(OH)j contcnt by titrating it with 0.250 M HC1. The volume of HC1 solution required to react with the aluminum hydroxide in the rock (assuming no other minerals in the rock were reactive towards HCl) was found to be 11.6 ml The content of aluminum hydroxide in the rock was: a. 7.7% b. 8.9% c. 13.1% d. 22.6% e. 23.2% f. 39.3% g. 69.6% h. None of the above; the correct contcnt was_%

Explanation / Answer

m = 0.3250 g

Al(OH)3 contianing

MW = 78.0036 g of Al(OH)3

M = 0.25 HCl

V = 11.6 ml of HCl needed

find % aluminium

first,

find mol ofHCl used

mol = MV = 0.25*11.6 = 2.9 mmol of acid used

that is, since ratio is 1:3

3HCl + Al(OH)3 = 3H2O + AlCl3

then

2.9/3 = 0.966666 mmol of Al(OH)3

mass = mol*MW = 78.0036*0.966666*10^-3 = 75.40*10^-3 g of Al(HO)3

then

% mass = mass of Al(OH)3 / total mass ¨* 100% = ( 75.40*10^-3)/(0.3250) *100= 23.2 %

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