[Wsx594]

34.05 g of an unstable liquid compound is put in a piston with a diameter of 20 cm. The piston is then placed in 15 kg of water at 30 degrees C. During the reaction the water temperature rises 4.52°C and the piston rises 59.1 cm . At the end of the reaction you have .45 mol CO2 (g) , .375 mol H2O (l) , and .0375 mol O2 (g) and an unknown amount of an of gas of an unknown element. Assume the heat capacity and the mass of the piston is negligible. The enthalpy of decomposition for unknown= -1893 Kj/mol. find the unidentified element.
To start I did q= (34.05 + 15000)x 4.52 x 4.184 which q=-284.047 Kj
where would I go from here?

[Borek]

I would try to find molar mass of the element. You should be able to use ideal gas equation to find out total number of moles of gas. That will give number of moles of the element. It also looks like there is enough information to find out its mass.

I feel like the decomposition enthalpy is just a red herring, can be wrong though.