When a solid dissolves in water, heat may be evolved or absorbed. The heat of di

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 0.78 g of MgCl2(s) are dissolved in 106.60 g of water, the temperature of the solution increases from 25.67 to 28.70 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be1.56 J/°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 equal to the specific heat of water.

Hdissolution = ___________kJ/mol

The following thermochemical equation is for the reaction of carbon dioxide(g) with hydrogen(g) to form carbon monoxide(g) and water(g).

When 15.1 grams of carbon dioxide(g) react with excess hydrogen(g),  kJ of energy are _________evolved or absorbed.

Hint: An amount of energy is expressed as a positive number. The sign indicates whether the energy is absorbed or evolved.

Explanation / Answer

I have a little doubt with the first question, but I'll answer the second one:

First, you need to calculate the moles:

moles of CO2 = 15.1 / (12+32) = 0.34 moles

These moles, according to the reaction and the fact that CO2 is the limitant reactant, are the moles produced in the whole reaction so:

Q = 41.2 * 0.34

Q = 14.01 kJ of energy evolved.

Hope this helps. For the first question, please post it in another question thread while I read a little about this problem.

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