A student investigates the dissolution of magnesium chloride in water

CHEMISTRY · Multi-part · A Level · Chemistry · Thermodynamics · Thermodynamics

Problem

A student investigates the dissolution of magnesium chloride in water. The balanced chemical equation for complete dissolution is

\text{MgCl}_{2}(s)\longrightarrow\text{Mg}^{2+}(aq)+2\,\text{Cl}^{-}(aq).

Under standard conditions the process as written for exactly

3.00\,\text{mol}

\text{MgCl}_{2}(s)

has

\Delta S^{\circ}=+261\,\text{J\,K}^{-1}

. (a) Determine the standard molar entropy change, in

\text{J\,mol}^{-1}\,\text{K}^{-1}

, for the dissolution of

1.00\,\text{mol}

\text{MgCl}_{2}(s)

. Express your answer to the correct number of significant figures. (b) Convert your result from part (a) to units of

\text{kJ\,mol}^{-1}\,\text{K}^{-1}

. (c) The student writes a new equation in which only

0.250\,\text{mol}

of solid dissolves. Calculate the corresponding

\Delta S^{\circ}

(in

\text{J\,K}^{-1}

) for this smaller-scale process.