# need help with Molar enthalpies

• Jan 19th 2009, 02:36 PM
supersaiyan
need help with Molar enthalpies
Sorry, if I am not allowed to ask non math related question

1) add 60 ml of 1.0 mol/L Sodium Hydroxide solution to a polystyrene calorimeter.
2) mix it with 40-ml sample of 1.0 mol/L sulfuric acid solution in a graduated cylinder.

3) Add the acid to the base, stirring slowly with the thermometer. record the highest temperature recorded

initial temp: 22.5
highest: 26

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Cacluate the number of moles of the base (the limiting reagent)

2) Calculate the change in molar enthalpy with respect to sodium hydroxide.

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I think, I solved it. n = c.v
n = 0.06 * 1 = 0.06 mol NaOH

2nd) H = m.c.t/n

100g * 4.184 * 3.5 / 0.06
= 24.4 KJ
is this correct?

the reason why I am confused is because the accepted value is 56kj/mol. My answers is way off. Wither I messed up during the experiment or in my writtten solution or in both
Plz help
• Jan 19th 2009, 02:54 PM
o_O
Your calculations are correct. But just looking at your data, there should've been a larger temperature change. A change of 3.5 degrees Celsius really isn't a lot. I remember I did this experiment with NaOH and HCl and we had a change of about 20 degrees Celsius.

Did you cover your calorimeter with a lid? Chances are a lot of heat escaped from it into the atmosphere due to the fact that the system is not closed (no lid/ poor insulated cup).