Potassium nitrate, KNO_3, has a molar mass of 101.1 g/mol. In a constant-pressur

Question

Potassium nitrate, KNO_3, has a molar mass of 101.1 g/mol. In a constant-pressure calorimeter, 37.4 g of KNO_3 is dissolved in 315 g of water at 23.00 degree C. KNO_3(s) + H_2O(aq) rightarrow KOH(aq) + HNO_3(aq) The temperature of the resulting solution decreases to 19.30 degree C. Assume the resulting solution has the same specific heat as water, 4.184 J/(g degree C). and that there is negligible heat loss to the surroundings. How much heat was released by the solution? What is the enthalpy of the reaction?

Explanation / Answer

Molar mass of KNO3 (Mw) = 101.1 g/mol

Mass of KNO3 (m1) = 37.4 g

Mass of water (m2) = 315 g

a)

Total mass of resulting solution, m = m1 + m2 = 37.4 + 315 = 352.5 g

Initial temperature of water, Ti = 23 C

Final temperature of water, Tf = 19.3 C

Specific heat of solution, Cp = 4.184 J/g C

Heat released by solution, Q = m * Cp * (Tf – Ti)

Q = 352.5 * 4.184 * (19.3 – 23) = -5455.4 J = -5.46 kJ

b)

Moles of KNO3 = m1/Mw = 37.4 / 101.1 = 0.37 moles

Heat absorbed by KNO3 = - Q

Enthalpy of the reaction = - Q / moles of KNO3

= -5.46 / 0.37 = 14.75 kJ/mol

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