A calorimeter contains 27.0 mL of water at 11.5 C . When 1.70 g of X (a substanc

Question

A calorimeter contains 27.0 mL of water at 11.5 C . When 1.70 g of X (a substance with a molar mass of 75.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 30.0 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

Tw = 11.5

m = 1.7 g of X; MW 75 g/mol

mol of X= mass/MW = 75/1.7 = 44.117 mol of X

dT = 30-11.5 = 18.5 °C

find dHRxn for this

HRXn = -Qrxn/mol

Qrxn = m*C*(Tf-Ti) = 27*4.184*18.5 = 2089.908 J

HRxn = -2089.908 / 44.117

HRxn = -47.37194 J/mol

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