Question

When 7.56 g of NaCl is added to a coffee cup calorimeter, the water temperature changes by 4.1 ºC. If the heat of solution (the enthalpy change upon dissolving in water) is 3.8 kJ/mol, what mass of solution must be in the cup? Assume the specific heat capacity of the solution is the same as the specific heat capacity of water.

Answer 1

Molar mass of NaCl,

MM = 1*MM(Na) + 1*MM(Cl)

= 1*22.99 + 1*35.45

= 58.44 g/mol

mass(NaCl)= 7.56 g

number of mol of NaCl,

n = mass/molar mass

=(7.56 g)/(58.44 g/mol)

= 0.129 mol

heat of solution = 3.8 KJ/mol

SO, heat released = number of mol * heat of solution

= 0.129 mol * 3.8 KJ/mol

= 0.4902 KJ

= 490.2 J

This heat is absorbed by solution which changes the temperature by 4.1 oC

Specific heat capacity for solution, C = 4.184 J/g.oC

Now use:

Q = m*C*delta T

490.2 = m * 4.184 J/g.oC * 4.1 oC

m = 28.6 g

Answer: 28.6 g