a calorimeter contains 33.0mL of water at 13.0 Celcius. when 2.50 g of X ( a sub

Question

a calorimeter contains 33.0mL of water at 13.0 Celcius. when 2.50 g of X ( a substance with a molar mass of 82.0g/mol) is added, it dissolves via the reaction

X(s)+H2O(L)--->X(aq)

and the temperature of the solution increases to 26.5 Celcius. Calculate the enthalpy change, DELTA 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

Mass of water = volume x density

= 33.0 x 1.00 = 33.0 g


Mass of solution = mass of water + mass of X

= 33.0 + 2.50 = 35.5 g


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

= 35.5 x 4.18 x (26.5 - 13.0) = 2003 J


Moles of X = mass/molar mass of X

= 2.50/82.0 = 0.03049 mol


Delta H = -heat released/moles of X

= -2003/0.03049

= -6.57 x 10^4 J/mol = -65.7 kJ/mol

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