Excess Enthelpy for Water-Ethanol Solution

[miz_cheme]

I am trying to determine the final temperature of a solution after I mix equal volumes of pure water and 95% by volume Ethanol (5% by volume water). Doing this experiment in the lab, I found that mixing 50 mL of water and 50 mL of 95% Ethanol gave a final temperature of 28.5 C after mixing (both the water and ethanol were at the same temperature of 22 C before mixing). Thus, mixing 50 mL of each when both were initially at 22 C gave in increase of 6.5 C.

With the mole fraction of Ethanol being about 0.2, this gives a corresponding excess enthalpy of -717 J/mol and with the total moles of Ethanol at 0.77, the added enthalpy after mixing is -553. This leads to a calculated temperature increase of only 1.7 C. What am I missing here? Why did the lab results show a temperature of 28.5 yet my math (which I am nearly 100% of correctness) only gives 1.7 C increase?

 

[Bystander]

What heat capacities are you using for components and for the mixture?