Question

The reaction nacl(s) → nacl(aq) is performed in a coffee cup calorimeter, using 100 ml of h2o(l) and 5.00g of nacl. if the temperature of solution after mixing decreased by 2.30°c, what is the δhrxn in kj/mol? assume the heat capacities of all solutions are 4.18 j/g°c, and densities of all solutions are 1.00 g/ml. the molar mass of nacl is 58.44 g/mol

Answer 1

Since the density of water is 1 g /mL, hence there is 100 g of H2O. So total mass is:

m = 100 g + 5 g = 105 g

 

=> The heat of reaction can be calculated using the formula:

δhrxn = m C ΔT

where m is mass, C is heap capacity and ΔT is change in temperature = negative since there is a decrease

 

δhrxn = 105 g * 4.18 J/g°C * (-2.30°C)

δhrxn = -1,009.47 J

 

=> However this is still in units of J, so calculate the number of moles of NaCl.

 

moles NaCl = 5 g / (58.44 g / mol)

moles NaCl = 0.0856 mol

 

=> So the heat of reaction per mole is:

δhrxn = -1,009.47 J / 0.0856 mol

δhrxn = -11,798.69 J/mol = -11.8 kJ/mol