Answer to Question #231009 in General Chemistry for Naz

Question

When 2.35g Mg(OH)2 is added to 250.0 mL of water, the temperature of the water raises from 20.5 oC to 36.0 oC. Calculate the molar enthalpy of solution.

Answer 1

Amount of energy absorbed by water:

$q=cm\Delta T = 4.186 \frac{J}{g\times ^\circ C} \times 250.0 g \times (36.0 ^\circ C-20.5 ^\circ C)= 16220.75 J$

Moles of solute:

$2.35 g Mg(OH)_2 \times (\frac{1 mole Mg(OH)_2}{58.32 g Mg(OH)_2})= 0.0403 mol$

As the temperature is increased then the reaction is exothermic, then:

$\Delta H_{soln}= \frac{-q}{n}$

$\Delta H _{soln} = \frac{-16220.75 J}{0.0403 mol}=-402500 \frac{J}{mol}=- 403 \frac{kJ}{mol}$