A calorimeter contains 28.0 mL of water at 15.0 C . When 1.60 g of X (a substanc
ID: 948116 • Letter: A
Question
A calorimeter contains 28.0 mL of water at 15.0 C . When 1.60 g of X (a substance with a molar mass of 44.0 g/mol ) is added, it dissolves via the reaction
X(s)+H2O(l)X(aq)
and the temperature of the solution increases to 25.0 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 lost by substance = - q gain by water
q of X = - q H2O
= - m* s * dT
= - ( 28 * 4.18 * (25 - 15))
q of X = -1170.4 J for 1.6 g of substance
for 44 g of X compound q = - 1170.4 * 44/1.6 = -32186 J / mol
q of X = -32.2 kJ / mol
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