empirical formula

Question

A 15.85 g sample of Mo2O3(s) is converted completely to another molybdenum oxide by adding oxygen. The new oxide has a mass of 16.91 g . 

Add subscripts to correctly identify the empirical formula of the new oxide.

Answer 1

To answer this question you need to determine the number of mol of Mo2O3 since the question states that all of Mo2O3 is converted to another molybdenum oxide. Therefore, the number mol will stay the same if one adds oxygen.

The molar mass is defined as g/mol. You have given both, the g (16.91) and the mol through Mo2O3.

To solve this issue you need to do the following:

1.    Determine the number of mol of 15.85g of Mo2O3: Molar Mass (MM) of Mo2O3 = 239.92 g/mol. mol = mass/Molar Mass; n =m/MM; n = 15.85g / (239.92g/mol) = 0.066mol

2.    Determine the molar mass of the newly formed molybdenum oxide: MM = g/mol = 16.91g / 0.066 mol = 255.96 g/mol.

3.    255.96 g/mol corresponds to Mo2O4 (Mo2O3 = 240 g/mol + O(Oxygen) = 16 g/mol = 256 g/mol).