momogeek2141 Posted February 4, 2015 Share Posted February 4, 2015 Benzoic Acid, C7H6O2, is a standard used in determining the heat capacity of a calorimeter. The standard entropy (Delta H degree sign) of combustion of Benzoic acid is 3.22 *10^3 kJ/mol. 0.5 g of benzoic acid was burned in a calorimeter containing 1000.0 g of water. The change in temperature of the calorimeter was 3 degrees celsius. Calculate the heat capacity of the calorimeter in J/K. A. 450 J/K B. 210 J/K C. 4025 J/K D. 2307 J/K I have no idea how to do this problem. Any help? Quote Link to comment Share on other sites More sharing options...
BBM Posted February 4, 2015 Share Posted February 4, 2015 (edited) guessing you mean the standard enthalpy of combustion and not entropy So benzoic acid has a molar mass of (7 x 12 + 6 x 1 + 16 x 2) = 122 g/mol. Burning 0.5g of it means you burn 0.5/126 = 0.004 mol. Multiplying that by the enthalpy of combustion, you get an energy release of ~13.2 kJ. Now, water has a specific heat capacity of ~4.2 kJ/kgK. If you assume that the temperature of the calorimeter and the water rise together, and it rose 3 K, 4.2 * 1 * 3 = 12.6 means that only around 0.6 kJ went into heating the calorimeter. Dividing that by 3 K gives you 0.2 kJ/K, which is pretty close to 210 J/K It's basically an energy balance- think of the law of the conservation of energy. You release some energy from combusting the benzoic acid, and assuming none is lost to the environment, that energy has to go towards heating up the water and heating up the calorimeter. Edited February 4, 2015 by BBM Quote Link to comment Share on other sites More sharing options...
momogeek2141 Posted February 4, 2015 Author Share Posted February 4, 2015 Ok thanks! And yes I did mean enthalpy, sorry about that. Quote Link to comment Share on other sites More sharing options...
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