Answered: When a solid dissolves in water, heat…

Chemistry: The Molecular Science

5th Edition

ISBN:9781285199047

Author:John W. Moore, Conrad L. Stanitski

Publisher:John W. Moore, Conrad L. Stanitski

Chapter4: Energy And Chemical Reactions

Section4.8: Measuring Reaction Enthalpies: Calorimetry

Problem 4.17E

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Question

When a solid dissolves in water, heat may be evolved or absorbed. The heat of dissolution (dissolving) can be determined using a coffee cup calorimeter.

In the laboratory a general chemistry student finds that when 7.63 g of K₂SO₄(s) are dissolved in 107.20 g of water, the temperature of the solution drops from 25.27 to 22.68 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.54 J/°C.

Based on the student's observation, calculate the enthalpy of dissolution of K₂SO₄(s) in kJ/mol.

Assume the specific heat of the solution is equal to the specific heat of water.

ΔH_dissolution =?? kJ/mol

Expert Solution

Step 1

Mass of water = 107.20g

Mass of K₂SO₄ = 7.63

Molar mass of K₂SO₄ = 174.27 g/mol

Mass of solution = (7.63 + 107.20) g

= 114.83 g

$\begin{array}{rcl} Moles of K_{2} {SO}_{4} & = & \frac{Mass}{Molar mass} \\ = & \frac{7.63 g}{174.27 g / mol} \\ = & 0.044 moles \end{array}$

Step 2

Heat released -

$\begin{array}{rcl} Heat released & = & - [(Mass of solution \times specific heat \times temperature difference) + (calorimeter costant \times temperature diffrenece)] \\ = & - [107.20 g \times 4.184 J / g^{\circ} C \times {(22.68 - 25.27)}^{\circ} C + 1.54 J /^{\circ} C \times {(22.68 - 25.27)}^{\circ} C] \\ = & 1165.67 J \end{array}$

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