Homework Help: chemistry

Posted by Dina on Thursday, December 3, 2009 at 11:05pm.

Can anyone help me with these Chemistry questions? I am trying to study for an upcoming test and its just not sinking in as well and I thought.

This is Thermochem for a heads up:

Suppose 33mL of 1.20M HCl is added to 42mL of a solution containing excess sodium hydroxide in a coffee-cup calorimeter. The solution temperature, originallu 25.0 degrees celcius, riases to 31.8 degrees C. Give the enthaply change for the reaction
HCl(aq) + NaOH(aq) -> NaCl(aq) + H20 (l)

Express the answer as a thermochemical equation. For simplicity, assume that the heat capcity and the density of the final solution in the cup are thoe of water. Also assume that the total volume of the solution equals the sum of the volumes HCl and NaOH.

The answers is -54kJ... but how?

Next...

What will the final temperature of a mixture made from 25.0 g of water at 15 degrees C, 45 g of water from 50 degrees C, and from 15 g of water at 37 degrees C?

Answer is 34.7 degrees C.

chemistry - Dr Russ, Friday, December 4, 2009 at 6:32am
Heat capacity of water is 4.2 J/g/degC

If density of water is 1 g/ml then the mass of water is 75 g.

the temperature change is 6.8 degC so the energy given out by the reaction (i.e. it is exothermic) is

4.2 J/g/degCx 75 g x 6.8 degC = 2.142 kJ

the 33ml of 1.20M HCl contains

33x1.20 /1000 moles = 0.0396 mole

so delta H is -2.142 kJ/0.0386 mole

=-54 kJ mol^-1

- because it is exothermic.

check my maths and my units!

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