Question

The salt magnesium chloride is soluble in water. When 0.630 g of MgCl₂ is dissolved in 114.00 g of water, the temperature of the solution increases from 25.00 to 27.29 °C. Based on this observation, calculate the enthalpy of dissolution of MgCl₂ (in kJ/mol).

Assume that the specific heat of the solution is 4.184 J/g °C and that the heat absorbed by the calorimeter is negligible.

ΔH_dissolution = ? kJ/mol

Answer 1

MgCl2 when dissolved in water evolved heat and due to this the temperature of the solution increased from 25 C to 27.29 C

Change in temperature = T = 27.29 C - 25 C = 2.29 C

H = mST

where m is the mass of the solution = mass of Mg + mass of water

= 0.63g + 114g = 114.63g

S is the specific heat of solution = 4.184 J/g-C

H is the amount of heat evolved in the process

Putting values in H we get

H = 114.63g*4.184J/g-C*2.29C = 1098.31 J

To find Hdissolution we need to divide H by no. of moles of MgCl2 used

molar mass of MgCl2 = molar mass of Mg + 2*molar mass of Cl

= 24.3g/mole + 2*35.45g/mole

= 95.2g/mole

moles of MgCl2 in 0.63g = weight/molar mass

= 0.63g/95.2g/mole = 0.006617 moles

Hdissolution = H/moles of MgCl2

= 1098.31 J/0.006617 moles

= 165983 J/mole

= 165983/1000 kJ/mole = 165.983 kJ/mole

As the heat is evolved, we put a - sign to denote this as exothermic process.

So, Hdissolution = -165.983 kJ/mole