Question

The concentration of arsenic in an insecticide is determined gravimetrically by precipitating MgNH4AsO4 and isolating Mg2As2O7. Determine the %w/w As2O3 in a 1.627-g sample of insecticide if it yields 106.5 mg of Mg2As2O7.

Answer 1

We first note that both As2O3 and Mg2As2O7 contain two arsenic atoms, so we will not need a ny extra conversion in the calculations.
310.4490 g Mg2As2O7 = 1 mol
0.1065 g of Mg2As2O7 = 0.000343 moles

(2) 1 mole As2O3 is equivalent to 1 mole of Mg2As2O7 (as stated above, each contains 2 moles of arsenic), so

0.000343 moles Mg2As2O7.= 0.000343 moles As2O3.
197.8 g As2O3   means one mole

0.000343 moles means =0.000343 x 197.8=0.0678454grmns


(4) calculate % As2O3 in sample:

% As2O3 = 100% x (__0.0678485_ g As2O3)/(1.627-g sample of insecticide) = __4.17__ % As2O3