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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Ice-cream on May 16, 2005, 01:23:30 AM

Title: calorimetry
Post by: Ice-cream on May 16, 2005, 01:23:30 AM
Can any1 help me with this?

In a coffee-cup calorimeter, 50ml of 0.1M AgNO3 and 50ml of 0.1M HCl are mixed to yield the following reaction:

Ag+(aq) + Cl-(aq) --> AgCl(s)

The 2 solutions were initially at 22.6 degrees C and the final temperature is 23.4 degrees C. Calculate the heat that accompanies this reaction in kJ/mol of AgCl formed. Assume that the combined solution has a mass of 100g and a specific heat capacity of 4.18 J/degrees C g.

I used q = ms(detaT) = 100 x 4.18 x 0.8 = 334.4J. I then thought since there are 0.01 mols altogether, then that means for 0.01 moles it's 334.4J, so 1 mole is 33440J. but that's the wrong answer. I think it's the moles part that's wrong - i'm not sure if it's ok to find moles of AgNO3, moles of HCl and then just add them up...

then answer's -66kJ/mol
Title: Re:calorimetry
Post by: eugenedakin on May 18, 2005, 10:58:21 PM
Hello Ice-cream,

You are REALLY close... here is what I calculated...

50 ml = 0.05 litres

0.05 litres x 0.1 M = 0.005 moles

0.005 moles Ag + 0.005 moles Cl => 0.005 moles AgCl

I agree with your calculation of 334.45 J

Take 334.45 J / 0.005 moles => 66890 J/mol (or 67 KJ/mol)

since the reaction is exothermic, it is a negative value (-67 KJ/mol)

Its not perfect, but I hope that it helps,