Question

An element, M, reacts with oxygen to give an oxide of formula M3O4. A sample of M of 257 mg is placed in a 500.0 mL vessel containing oxygen at 0.991 atm and 23.0°C. After cooling the vessel back to the original temperature, the pressure was 0.813 atm. Calculate the molar mass of M.

I don't know what to do about the 2 different pressures given... do I subtract the final one from the inital one and use that number? (in the Ideal Gas Law?)

Answer 1

Yes, subtract 0.991 atm - 0.813 atm = ?? The difference is the amount of oxygen that reacted. Use PV = nRT then to determing the mols of oxygen. From that and the formula you can determine the mols of M and that with the 257 mg M will give you the molar mass of M.