Adding 5.44 g of NH_4NO_3(s) to 150.0 g of water in a coffee-cup calorimeter (wi

Question

Adding 5.44 g of NH_4NO_3(s) to 150.0 g of water in a coffee-cup calorimeter (with stirring to dissolve the salt) resulted in a decrease in temperature from 18.6 degreeC to 16.2 degreeC. Calculate the enthalpy change for dissolving NH_4NO_3(s) in water, in kj/mol. Assume the solution (whose mass is 155.4 g) has a specific heat capacity of 4.2J/g middot K. (Cold packs take advantage of the fact that dissolving ammonium nitrate in water is an endothermic process.) A cold pack uses the endothermic enthalpy of a solution of ammonium nitrate.

Explanation / Answer

m = 5.44 g of NH4NO3

mol = mass/MW = 5.44/80.05 = 0.06795 mol

m = 150 g of water

dT = 16.2-18.6 = -1.8 °C, which is endothermic since dt < 0

Hsoln = -Q/n

Q = m*C*(Tf-Ti) = (150+5.44)*4.184*(-1.8) =-1170.649 J

Hsoln = -Q/n = -(-1170.649 ) / (0.06795 ) = +17228.09 J/mol

Hsokn = 17.23 kJ/mol

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