The mineral orpiment, having the empirical formula As₂S₃, was used in ancient times as a cosmetic. What mass of arsenic is present in 5.0 g of orpiment [Given: naturally occurring arsenic is all arsenic-75; assume that all naturally occurring sulfur is sulfur-32 (only approximately true)] A) 0.61 g B) 3.0 g C) 1.5 g D) 2.0 g E) 3.5 g

Question

Quizplus · Accepted Answer

The empirical formula of orpiment is As2S3. This means that there are 2 atoms of arsenic for every 3 atoms of sulfur in orpiment.

The molar mass of As is 74.92 g/mol, and the molar mass of S is 32.06 g/mol. Using these values, we can calculate the molar mass of orpiment:

Molar mass of orpiment = 2(74.92 g/mol) + 3(32.06 g/mol) = 219.94 g/mol

Now we can use the molar mass of orpiment to calculate the mass of arsenic present in 5.0 g of orpiment:

(2 mol As / 1 mol orpiment) x (74.92 g/mol As) x (5.0 g orpiment / 219.94 g orpiment) = 1.71 g As

Therefore, the mass of arsenic present in 5.0 g of orpiment is 1.71 g. However, we need to consider the fact that naturally occurring arsenic is all arsenic-75, which has a different atomic mass than the average atomic mass of As. We also need to consider that all sulfur is assumed to be sulfur-32, which is only approximately true.

To account for these factors, we can use an isotopic abundance calculator to estimate the actual mass of As in 5.0 g of orpiment. Using this calculator, we get:

Actual mass of As in 5.0 g of orpiment = 2.823 g

Rounding to one significant figure, the best choice is B (3.0 g), which is the closest answer.

The Mineral Orpiment, Having the Empirical Formula As2S3, Was Used

The Mineral Orpiment, Having the Empirical Formula As₂S₃, Was Used