Find the percentage As_2O_3 in a sample.

Question

A sample containing As (74.92 g/mol) weighs 0.1500 grams. The sample is dissolved in HNO_3, forming H_3AsO_4 The resulting solution is made neutral and then treated with AgNO_3, which precipitates all the As as Ag_3AsO_4. The precipitate is washed and dissolved in acid. The Ag in solution is precipitated with excess chloride ion, yielding a precipitate of AgCl (143.32 g/mol) which is weighed. What is the percentage of As_2O_3 (197.84 g/mol) in the sample if the precipitate weighed 0.0156 grams?

Answer 1

first let's find the only moles we can , AgCL whose molar mass is 143.32 g/mol:

0.0156 grams AgCl @ 143.32 g/mol = 1.088e-4 moles AgCL

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arsenic is ppt't : 3 Ag+ & 1 AsO4-3 --> 1 Ag3AsO4

then silver is ppt'd : 3 Ag+ & 3 Cl- --> 3 AgCL

we see that 1 mole of AsO4)-3 produces 3 moles of AgCl.

so use that ratio to find the moles of AsO4)-3 :

1.088e-4 moles AgCl @ 1 mol AsO4)-3 / 3 moles AgCl = 3.63e-5 moles AsO4)-3

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now lets find the moles of As2O3 which has 2 As that produced the AsO4)-3 that has 1 As (2:1 ratio):

3.63e-5 mol AsO4)-3 @ 1 mol As2O3 / 2 mol ASO4)-3 = 1.81e-5 mol As2O3

so lets find the weight of that As2O3 using its molar mass:

1.81e-5 mol As2O3@ 197.84 g/mol = 3.59e-3 grams of As2O3

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now the %:

3.59e-3 g As2O3 / 0.1500 g sample (100) = 2.39 %

your answer is : 2.39%

its a pretty small % of As, because it was a very small AgCl ppt