When 2.950 g of AX (s) dissolves in 100.0 g of water in a coffee-cup calorimeter

Question

When 2.950 g of AX (s) dissolves in 100.0 g of water in a coffee-cup calorimeter the temperature rises from 22.0 oC to 31.7 oC.  Calculate the enthalpy change (in kJ/mol) for the solution process

AX (s) --> A+ (aq) + X- (aq)

Assumptions for this calculation:

The specific heat of the solution is the same as that of pure water (4.18 J/ g-K).

The density of water = 1.000 g/mL.  The liquid's final volume is not changed by adding the solid.

The calorimeter loses only a negligible quantity of heat.

The formula weight of AX = 52.7398 g/mol.  Make sure you include the correct sign for the enthalpy change.  The units are kJ/mol.  

Explanation / Answer

Heat released = mass x specific heat x temperature change of solution

= (100.0 + 2.950) x 4.18 x (31.7 - 22.0) = 4174.2 J


Moles of AX = mass/molar mass of AX

= 2.950/52.7398 = 0.055935 mol


Enthalpy of solution = -heat released/moles of AX

= -4174.2/0.055935

= -74626 J/mol

= -74.63 kJ/mol = -74.6 kJ/mol

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