A calorimeter contains 24.0 mL of water at 15.0 C . When 2.30 g of X (a substanc

Question

A calorimeter contains 24.0 mL of water at 15.0 C . When 2.30 g of X (a substance with a molar mass of 57.0 g/mol ) is added, it dissolves via the reaction

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

and the temperature of the solution increases to 26.0 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/(gC)], that density of water is 1.00 g/mL, and that no heat is lost to the calorimeter itself, nor to the surroundings.

Express the change in enthalpy in kilojoules per mole to three significant figures.

Explanation / Answer

q = mCT

m = density * volume = 1 g/ml * 24 mL = 24 g

q = 24 g * 4.18 J/g-deg * ( 26 - 15) deg

q = 24 g * 4.18 J/g-deg * 11 deg

q = 1104.58 J

q = 1.104 kJ

moles of X = given mass / Molar mass = 2.30g / 57 g/mol = 0.0404 moles

H = 1.104 kJ / 0.0404 mol = 27.3 kJ / mol

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