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8.99 math

Posted: Tue Feb 06, 2018 2:54 pm
by Jessica Wakefield 1H
Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. Apiece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800 mL of 0.500 M HCL(aq). If the initial temperature of the hydrochloric acid solution is 25*C, what is the final temperature of this solution? Assume that the density and molar heat capacity of the hydrochloric acid solution are the same as those of water and that all the heat is used to raise the temperature of the solution.

The solutions manual find the enthalpy of the reaction first (-20000 J) and sets it equal to (-4.184)(800 g)(dT)

I was wondering where the 800 g came from because it says that the zinc is 8.5 g and the HCl is .130 mol

Re: 8.99 math

Posted: Tue Feb 06, 2018 4:56 pm
by Matthew Lin 2C
The 800 g comes from the mass of the solution. You know this because the problem says that the density of the solution is the same as water (1 g/mL) and since the solution has a volume of 800 mL, then it has a mass of 800 g. Hope this helps!

Re: 8.99 math

Posted: Wed Feb 07, 2018 11:34 am
by Pooja Nair 1C
The reason that 0.130 mol Zinc is relevant is because the enthalpy of formation is kJ per mole. Therefore, when you find the energy of the reaction, you have to find the amount of energy used to carry the reaction to completion with 0.130 mol of Zinc. The 800 grams comes from the volume of the HCl, which is 800 g/L