A 2.00-g sample of a salt is dissolved in 30.0 g of distilled water in a calorim

Question

A 2.00-g sample of a salt is dissolved in 30.0 g of distilled water in a calorimeter. The temperature of the solution rises from 20.5 ºC to an extrapolated final temperature of 31.7 ºC. Assume that the mass of the solution is the combined masses of the salt and the water and that the specific heat capacity of the solution (C) is 4.18 J/g.ºC. In this process, the solution represents the surroundings and the reaction represents the system.

Use the information and equation provided in the experiment handout to answer the following questions:

qsurr=sol = m x C x (Tf – Ti) and qsys=rxn = - qsurr=sol

a) Calculate the heat of of the solution (surroundings), qsur in Joules. Clearly show ALL your calculations.

b) Determine the heat of the reaction of dissolution (system), qsys=rxn in Joules. Clearly show your reasoning.

c) Calculate the heat of the reaction of dissolution of this salt (M.M. = 121 g/mol) in kJ/g then in kJ/mol. Clearly show ALL your calculations.

Explanation / Answer

a)

mass of water = 30.0 g

Cp = 4.18 J / goC

temperature rise = 31.7 - 20.5 = 11.2 oC

Q = m Cp dT

    = 30 x 4.18 x 11.2

Q = 1404.5 J

heat of solution = 1404.5 J

b)

heat of the reaction of dissolution = - 1404.5 J

c)

moles of salt = 2 / 121 = 0.0165 mol

delta H = - Q / n

           = - 1404.5 x 10^-3 / 0.0165

delta H = - 84.97 kJ/mol

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