Hello Theo: Residual reactivity/loss of plasticizer is not usually the cause of polyurethane (PU) degradation as most PUs can get by with little or no small molecule additives. My strong suspicion
I haven't used a calorimeter since my college days. Did you take into account the heat gained by the calorimeter? A bigger sample should give you a bigger temperature change and help with errors due
Normally I do not help with homework, but I would be happy to give you the method: 1. find out which is the limiting reagent (LR) - convert both the calcium sulfide and the calcium sulfate to moles