Question

1. A student determines the heat of dissolution of solid magnesium chloride using a coffee-cup calorimeter of negligible heat capacity.

When 0.430 g of MgCl2(s) is dissolved in 118.00 g of water, the temperature of the solution increases from 25.00 to 26.54 °C. Based on the student's observation, calculate the enthalpy of dissolution of MgCl2(s) in kJ/mol. Assume the specific heat of the solution is 4.184 J/g°C. ___?

Answer 1

Even before doing any calculations, you can look at the change in temperature measured for the water to say that you can expect the standard molar enthalpy of dissolution of MgCl2 to be Negative.

Since adding the magnesium chloride salt to the water results in a increase in temperature, you can conclude that the dissolution of magnesium chloride release heat from the surroundings → you're dealing with an exothermic reaction.

So, your strategy here will be to use the mass of the water, its specific heat, and its change in temperature to determine how much heat was absorbed by the dissolution reaction.

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