A calorimeter contains 17.0mL of water at 12.0?C . When 2.00g of X (a substance

Question

A calorimeter contains 17.0mL of water at 12.0?C . When 2.00g of X (a substance with a molar mass of 69.0g/mol ) is added, it dissolves via the reaction

X(s)+H2O(l)?X(aq)

and the temperature of the solution increases to 26.5?C .

Calculate the enthalpy change, ?H, for this reaction per mole of X.

Assume that the specific heat of the resulting solution is equal to that of water [4.18 J/(g??C)], that density of water is 1.00 g/mL, and that no heat is lost to the calorimeter itself, nor to the surroundings.

Explanation / Answer

X(s) + H2O(l) => X(aq)

Mass of water = volume x density

= 17.0 x 1.00 = 17.0 g

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

= (17.0 + 2.00) x 4.18 x (26.5 - 12.0)

= 1151.59 J

Moles of X = mass/molar mass of X

= 2.00/69.0 = 0.028986 mol

Delta H = -heat released/moles of X

= -1151.59/0.028986

= -39730 J/mol = -39.7 kJ/mol

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