The standard temperature and pressure are 672mlO2, or 0.03mol, which just happens to react, so the molar number of XY2 is 0.0 1mol.

Before and after the reaction, the amount and volume of gas remain unchanged.

Therefore, according to the law of conservation of mass, the mass of XY2 is the mass after the reaction minus the mass of oxygen. That is, 2.56 * 0.672-0.03 * 32 = 0.76032g

The molar mass of XY2 is 0.72032/0.01= 72.032g/mol.