Question

The salt magnesium chloride is soluble in water. When 0.710 g MgCl₂ is dissolved in 111.00 g water, the temperature of the solution increases from 25.00 °C to 27.47 °C. Based on this observation, calculate the dissolution enthalpy, Δ_dissH, of MgCl₂.

Assume that the specific heat capacity of the solution is 4.184 J g^-1 °C^-1 and that the energy transfer to the calorimeter is negligible.

Answer 1

use,
heat absorbs by solution = m*s*(T2-T1)
mass of solution = (0.710+111.00) g
= 111.71 g
heat absorbs by solution = 111.71*4.184*(27.47 - 25.00)
= 1154.5 J

molar mass of MgCl2 = 95.2 g/mol
number of mole of MgCl2 = (given mass)/(molar mass)
= 0.710/95.2
= (7.46*10^-3) mole

the heat absorbs by solution is gained by heat released by the dissociation of MgCl2
so,
(7.46*10^-3) mole of MgCl2 releases 1154.5 J of energy
1 mole of MgCl2 releases (1154.5)/(7.46*10^-3) J of energy
1 mole of MgCl2 releases (154.8*10^3) J of energy
so,
(delta H)diss = (154.8*10^3) J/mo
= 154.8 KJ/mol

Answer : 154.8 KJ/mol