P x V = n x R x T
1.00 x V = 3.50 x 0.082 x 273
1.00 x V = 78.351
V = 78.351 / 1.00
V = 78.351 L
To do this problem it is necessary to take into account that the heat given by the unknown substance is equal to the heat absorbed by the water, but considering the correct sign:

Clearing the specific heat of the unknown substance:

Paramecia have a micro nuclei and a macro nuclei.