Question

1.057 grams of a substance with a molar mass of 385.336 g/mol are dissolved in 249.92. Ther temperature for the system changes from 83.83 to 52.38 degrees C during the mixing. What is the DH(heat of solution) for this process in kJ/mol. Assume that th density of water is 1.00g/mL.

I know that the heat capacity is 4.184 J/gK. I calculated the moles of substance and got 0.0027. I found the change in temperature is -31.45 degrees C. Where do I go from here?

Answer 1

q = mass x specific heat x delta T.
but we have a problem.
1.957 g .......are dissolved in 239.92 (what units and what is it dissolved in). My presumption is water but you need to say that.
IF it is 249.92 g H2O, then
the first equation is what you need.
You have two choices. You may use mass H2O in grams, specific heat is already in J/g and delta T you have. Then q will be J. Divide that by the mols of the substance and divide all that by 1000 to convert to kJ and you will have kJ/mol. Post your work if you get stuck.

Answer 2

Sorry about the units. It is 249.92mL of H20. Since the water is not in grams, what equation should I use?

Answer 3

Sorry my friend Barbie and I are doing are chem h.w. together. We each posted a question and since were using the same computer the last name always stays saved in the page...sorry I forgot to change it. It

Answer 4

The problem gives the density of water as 1.00 g/mL; therefore, 249.92 mL = 249.92 grams water. And in my first reponse I gave you only one choice. I can give you the other one but it is more complicated.

Answer 5

Should I use the mass of water, 249.92g, x 4.184j/gk x -31.45 divided by 1000? I did that and got -32.886.
I'm confused though, where does the 1.057g come into play? Also can my number be negative? Am I supposed to change the temperatures into Kelvin?

Answer 6

You do not need to change T to Kelvin. Since a 1 degree change in C is the same as a 1 degrees change in K, you are taking the difference between two temperatures, so you will get the same answer as a difference whether you use K or C.
Here is the way the problem works. You had so many g of a substance that dissolved in water. The equation I wrote first is the way to determine how much energy was spent in dissolving the substance.
q = massH2O x specific heat H2O x delt T for H2O = ?? Joules of heat (in this case lost since the water got colder).
Take that q in Joules, and do this.
a. divide by the mols of the substance. You have the grams and you have molar mass so you can get the mols. (mols = grams/molar mass). q/mol= the amount of heat lost per mole of the substance. Then divide by 1000 to change to kJ/mol and you have it. The sign for the answer is + since heat was absorbed by the substance (where did the heat come from---from the water, of course. The water got colder because heat was extracted from it to go into the substance for it to dissolve). I hope this helpsl I usually don't go into this much detail.