When 1.51 g of NH4Cl are dissolved in 100. mL of water at 25.00C, the temperature drops to 23.98C. Calculate the molar enthalpy of dissolving (kJ/mol)

Answer 1

To calculate the molar enthalpy of dissolving, we can use the equation:

q = m∙C∙ΔT

Where:
- q is the heat absorbed or released in the reaction (in J)
- m is the mass of the substance being dissolved (in g)
- C is the specific heat capacity of water (4.184 J/g°C)
- ΔT is the change in temperature (in °C)

First, let's calculate the heat absorbed by the water:

q = (100. g)(4.184 J/g°C)(23.98°C - 25.00°C)
q = -200.84 J

Since the temperature drops, the heat is released by the system.

Next, we need to calculate the number of moles of NH4Cl:

moles = mass / molar mass
moles = 1.51 g / (14.01 g/mol + 1.01 g/mol + 4(1.01 g/mol))
moles = 0.1 mol

Now, let's calculate the molar enthalpy of dissolving:

ΔH = q / moles
ΔH = -200.84 J / 0.1 mol
ΔH = -2008.4 J/mol

The molar enthalpy of dissolving is -2008.4 J/mol. To convert it to kJ/mol, divide by 1000:

ΔH = -2008.4 J/mol / 1000
ΔH = -2.0084 kJ/mol

Therefore, the molar enthalpy of dissolving NH4Cl is approximately -2.0084 kJ/mol.