In this example, we find the enthalpy of solution from experimental data using a coffee cup calorimeter. Our problem reads, "When 20.0 grams of potassium perchlorate is dissolved in 380.0 grams of water in a coffee cup calorimeter, the temperature changes from 21.05 degrees Celsius to 16.64 degrees Celsius. Calculate the enthalpy of solution for potassium perchlorate in kilojoules per mole. Use C equals 4.18 joules per gram degree Celsius for the specific heat capacity of the solution." Our first step is to calculate the heat released by the contents of the coffee cup. We can use the equation: heat equals mass times specific heat capacity times change in temperature. The total mass inside the coffee cup calorimeter is the 20.0 grams of potassium perchlorate plus 380.0 grams of water. Our change in temperature is the final temperature minus the initial temperature, which is 16.64 degrees Celsius minus 21.05 degrees Celsius. Completing the calculation, the contents of the coffee cup calorimeter released 7,374 joules. We know energy was released because the sign of heat is negative. Our next step is to calculate the thermal energy absorbed by the dissolving process. We know the dissolving process absorbs thermal energy because when an ionic solid is dissolved in water and the temperature decreases, the dissolving process is endothermic. The temperature is decreasing because thermal energy is being absorbed by the dissolving process, which means some of the thermal energy from the water is being converted into potential energy in the resulting solution. So, the energy absorbed by the dissolving process has the opposite sign of the energy released by the calorimeter's contents. Q solution should equal positive 7,374 joules. Our last step is to convert to kilojoules per mole of potassium perchlorate. We did not use one mole of potassium perchlorate in this experiment; we used 20.0 grams. So, we'll start our dimensional analysis with the heat that was absorbed, which is positive 7,374 joules, per 20.0 grams of potassium perchlorate. We can convert from joules to kilojoules using the definition of kilo. And we can use the molar mass of potassium perchlorate to convert from grams of potassium perchlorate to moles of potassium perchlorate. Completing the calculation, we get positive 51.1 kilojoules per mole of potassium perchlorate as the enthalpy of solution. The positive enthalpy of solution indicates the dissolving process is endothermic.