A calorimeter contains 26.0 mL of water at 12.0 C . When 1.80 g of X (a substanc

Question

A calorimeter contains 26.0 mL of water at 12.0 C . When 1.80 g of X (a substance with a molar mass of 70.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 26.0 C . Calculate the enthalpy change, H, for this reaction per mole of X. Assume that the specific heat of the resulting solution is equal to that of water [4.18 J/(gC)], that density of water is 1.00 g/mL, and that no heat is lost to the calorimeter itself, nor to the surroundings. Express the change in enthalpy in kilojoules per mole to three significant figures.

Explanation / Answer

Q = m*C*(Tf-Ti)

Q = (26+1.8)*4.18*(26-12) =1626.856 J

then

HRxn = -q/n = -1626.856 / (1.8/70) = -63266.6222 J/mol

HRxn = -63.33 kJ/mol

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