Question

When a 4.00-g sample of solid ammonium nitrate dissolves in 60.0 g of water in a coffee-cup calorimeter (see figure below), the temperature drops from 23.0°C to 16.4°C. Calculate ΔH (in kJ/mol NH4NO3) for the solution process shown below. Assume that the specific heat of the solution is the same as that of pure water. Hint: this process occurs at constant pressure.

Answer 1

first find out the Q using the following formula

Q = m x c x delta T

m = mass 60 + 4 = 64 gram

c = specifiv heat of the solution which is given as equal to water = 4.18 J/gC

delta T = 23 - 16.4 = 6.6 ºC

put all these in above equation

Q = 64 x 4.18 x 6.6

= 1765.632 Joules or 1.765 kJ

no of moles of ammonium nitrate = weight / molar mass

= 4 / 80 = 0.05 moles

when 0.05 moles dissolven in water produces 1.765 kJ

one mole will produce = 1.765 / 0.05 = -35.3 kJ