Help with simple calorimeter question

[Vindication]

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Hi All,

Don't be too hard on me but I need help with a simple bomb calorimeter question:

20 grams of NaCl is poured into a coffee cup calorimeter containing 250 ml of water. if the temperature inside the calorimeter drops 1 degree C by the time the NaCl is totally dissolved, what is the heat of solution for Nacl and water? (specific heat is 4.18 j/g C degree)


So I'm using the EK books to study with and this is question number 97 in the 7th edition of the EK chem book.

I looked up the solution and am not sure why they used 250 grams of water in the q=mc[]t equation. Why didn't they use grams of NaCl? maybe someone can offer a better explanation than the one provided. When do I use grams of water as the mass and when do I use grams of the compound? Thanks for your help

 

[PiBond]

Hi All,

Don't be too hard on me but I need help with a simple bomb calorimeter question:

20 grams of NaCl is poured into a coffee cup calorimeter containing 250 ml of water. if the temperature inside the calorimeter drops 1 degree C by the time the NaCl is totally dissolved, what is the heat of solution for Nacl and water? (specific heat is 4.18 j/g C degree)


So I'm using the EK books to study with and this is question number 97 in the 7th edition of the EK chem book.

I looked up the solution and am not sure why they used 250 grams of water in the q=mc[]t equation. Why didn't they use grams of NaCl? maybe someone can offer a better explanation than the one provided. When do I use grams of water as the mass and when do I use grams of the compound? Thanks for your help

calorimetry is used to determine the change in temperature of a mixture (solution) so we use the mass of the liquid when calculating the change in temperature. if the solution become colder then we know that the reaction involved was endothermic (required input of heat provided by solution) and if the solution got hotter then the reaction was exothermic and gave off heat. this ultimately helps us to obtain enthalpy values

because we're concerned with the delta T so much we want to make sure we don't lose any heat to the environment so it's better to use an insulating container

 

[MCAT4life]

your trying to find change in temp of the surroundings aka solution. And since it is water 250ml of water = 250 grams ... Caveat this is only for water. Since ml and grams are different dimensions. But since the density of water is 1 g/ml or 1 kg/liter, you can say that 250ml water = 250 grams. this is why i don't like the 1001 questions, their explanations are lacking as compared to TBR.

 

[Ahhdumb]

shouldn't the mass be 270g because the solution in question in Aqeuous NaCl?