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.

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.