A calorimeter contains 30.0 mL of water at 12.0 C . When 1.40 g of X (a substanc
ID: 904157 • Letter: A
Question
A calorimeter contains 30.0 mL of water at 12.0 C . When 1.40 g of X (a substance with a molar mass of 70.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 30.0 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
V = 30 ml of water
m = 30 g of water
T1 = 12 °C
m = 1.40 g of X
MW = 70
X + H2O --> X
Tf = 30.0
dH
Cp = 4.18
Q = m*Cp(Tf-Ti)
Q = (30+1.4) * 4.18 * (30-12) = 2364.8 J
mol = mass/MW = 1.4 / 70 = 0.02 mol
Qwin = -Qloss
then
HRxn = Q/n = -2364.8 / 0.02 = 118240J/mol or -118.24 kJ/mol
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