A calorimeter contains 20.0 mL of water at 12.5 C . When 1.50 g of X (a substanc

Question

A calorimeter contains 20.0 mL of water at 12.5 C . When 1.50 g of X (a substance with a molar mass of 58.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 26.5 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/(gC)], 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

Heat released by X raises the temperature of water and X from 12.5 oC to 26.5 oC
Mass of water = Volume * density = 20 mL*1 g/mL = 20 g
mass of x =1.5 g
Heat required = (m1+m2)*C*(Tf-Ti)
                              = (20+1.5)*4.18*(26.5-12.5)
                              =1258.18 J
1.5 g of X releases 1258.18 J
so,
1 mol or 58 g will release = 1258.18*58/1.5 = 48650 J = 48.65 KJ

so, H = 48.65 KJ/mol

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