A 10.0 g sample of calcium chloride is dissolved in 100. g of water (both at 20.
ID: 540349 • Letter: A
Question
A 10.0 g sample of calcium chloride is dissolved in 100. g of water (both at 20.0o C) in a light-weight calorimeter with negligible heat capacity. The final temperature is 36.0o C. Determine the molar enthalpy of dissolution of CaCl2(s) using two possible approaches:
a) assuming that the specific heat capacity of resulting solution (110. g) is the same as that of pure water (4.18 J/g-K) and that the entire solution participates in heat exchange rather than water alone;
b) assuming that the heat capacity of resulting solution is mostly due to that of water present (100. g) and that the heat capacity of CaCl2(aq) is negligible.
Solve the problem using both approaches. Note that answers in approaches (a) and (b) will be about 10% different from each other, and the true answer is somewhere in between.
Explanation / Answer
a)
the heat equation:
Q = m*C*(Tf-Ti)
Q = 110*4.18*(36-20)
Q = 7356.8 J
b)
Q = m*C*(Tf-Ti)
Q = 100*4.18*(36-20)
Q = 6688J
clearly lower, since we are not accounting those 10 g of reagents
% = (6688 - 7356.8 ) / 7356.8 *100 = 9% approx
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