Part A
A calorimeter contains 35.0 mL of water at 12.5 ∘C . When 2.10 g of X (a substance with a molar mass of 79.0 g/mol ) is added, it dissolves via the reaction
X(s)+H2O(l)→X(aq)
and the temperature of the solution increases to 28.0 ∘C .
Calculate the enthalpy change, ΔH, 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.
Express the change in enthalpy in kilojoules per mole to three significant figures.
Answer – Given, volume of water = 35.0 mL, mass of water = 35.0 g, since density = 1.0 g/mL mass of X = 2.10 g
Ti = 12.5oC , tf = 28.0oC
We know,
Heat loss = heat gain
Heat gain by water
q = m*C*∆t
= 35.0 g * 4.184 J/goC*(28-12.5)oC
= 2270 J
So, heat loss by X = 2270 J
= 2.270 kJ
Moles of X = 2.10 g / 79.0 g.mol-1
= 0.0266 moles
So, ∆H = -q
= -2.270 kJ / 0.0266 moles
= - 85.4 kJ/mol
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