chem30
posted by redeye .
what is the molar enthalpy of combustion for magnesium? calimetry experiment
results 100.0ml of HCl with 0.31g of Mg ribbon temp before ribbon added 23degrees and after ribbon added 37degrees. Need to show work so I understand. q=mcdeltaT

Mg + 2HCl ==> H2 + MgCl2
q = mass H2O x specific heat H2O x (TfinalTinitial)
q = 100 x 4.184 x (3723) = ? for reaction.
q/g Mg = q/0.31g
If you want to convert to g/mol, then
(q/0.31)*atomic mass Mg = ?
Ususually these are quoted in kJ/mol; convert to kJ by dividing by 1000. 
so i use the specfic heat of water even thought its 100.0ml Of HCl solution that the Mg ribbon is going into. What is the units for the 100 if I do crossing off what do I end up with the deltaH should be in kj/mol

The problem didn't say it but usually solutions of acids are assumed to have the same specific heat as H2O (since most of the solution is H2O anyway). As for the mL thing, again most problems assume the density of dilute acid solutions (again, most of it is H2O) is 1.00 g/mL; therefore, 100 mL has a mass of 100 grams.
The units look like this.
100 g x 4.184 J/g*C x (37 C  23C). The answer is in joules.
By the way, since it is delta H you want for kJ/mol Mg you must add a  sign to the answer. The water is there as a measuring stick for how much head is liberated by the reaction so the rxn liberates heat and the water absorbs it. q for the H2O comes out to be positive since it is absorbing the heat and that makes us add a  sign to q for the Mg rxn.
q/0.31 gives joules/gram.
(q/0.31g) x 24.3 g/mol gives J/mol and divide by 1000 to obtain kJ/mol Mg.