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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Ice-cream on May 12, 2005, 11:08:27 AM

Title: Enthalpy, heat, Raoult's law
Post by: Ice-cream on May 12, 2005, 11:08:27 AM
hey guys, i need some help with some questions:

1. Naphthalene combustion can be used to calibrate the heat capacity of a bomb calorimeter. The heat of combustion of naphthalene (C10H8) is 40.1kJ/g. When 0.8210g of naphthalene was combusted in a calorimeter containing 1000g water, a temperature rise of 4.21 degrees C was observed. What is the heat capacity of the bomb calorimeter excluding the water?

(What i've got so far is that since heat of combustion is 40.1 kJ/g, so 401 x 0.8210 = 329.221kJ must have been released by  naphthalene. Next i think i should use q = ms(delta T) except i'm not sure how we're meant to find the heat capacity of the bomb calorimeter "excluding the water"...can anyone help me out?)

2. Given the following information,
CaO(s) + H20 (l) --> Ca(OH)2(s)   delta H = -64.8kJ/mol
How many grams of CaO must react in order to liberate 504kJ of heat?

(I got 7.78 mols of CaO which means m = 435.56g. Does any1 agree?)

3. Thyroxine, an important hormone that controls the rate of metabolism in the body, can be isolated from the thyroid gland. If 0.455g of thyroxine is dissolved in 10.0 g of benzene, the freezing point of the solution is 5.144 degrees C. Pure benzene freezes at 5.444 degrees C and has a value for the molal freezing point depression constant Kf of 5.12 degree C kg mol^(-1). What is the molecular weight of thyroxine?

(I know to use delta Tf = Kf x mass of solute...but i don't understand how to find the mols of benzene so that i can find its molecular weight.)

Thanx
Title: Re:Enthalpy, heat, Raoult's law
Post by: Borek on May 12, 2005, 11:37:40 AM
i'm not sure how we're meant to find the heat capacity of the bomb calorimeter "excluding the water"...can anyone help me out?

Naphtalene heated both water and calorimeter - heat capacity of the heated system is sum of both heat capacity of water and heat capacity of calorimeter.

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I got 7.78 mols of CaO which means m = 435.56g. Does any1 agree?

Yes, I do.

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I know to use delta Tf = Kf x mass of solute...but i don't understand how to find the mols of benzene so that i can find its molecular weight.

You have 10.0 g of benzene - where is the problem in finding mole number? Besides it is not Kf * mass of solute but Kf * molality.
Title: Re:Enthalpy, heat, Raoult's law
Post by: GCT on May 13, 2005, 12:52:24 PM
hey guys, i need some help with some questions:

1. Naphthalene combustion can be used to calibrate the heat capacity of a bomb calorimeter. The heat of combustion of naphthalene (C10H8) is 40.1kJ/g. When 0.8210g of naphthalene was combusted in a calorimeter containing 1000g water, a temperature rise of 4.21 degrees C was observed. What is the heat capacity of the bomb calorimeter excluding the water?

Heat of reaction= -(heat_calorimeter+heat_water).

The heat capcity of the calorimeter is the heat absorbed by the calorimeter during the reaction.  You'll need to calculate the total heat due to the reaction, any of this heat energy not used in elevating the water temperature will have to gone to the calorimeter.

Using the heat_calorimeter, find the heat capacity (you should have been provided a formula for this).

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2. Given the following information,
CaO(s) + H20 (l) --> Ca(OH)2(s)   delta H = -64.8kJ/mol
How many grams of CaO must react in order to liberate 504kJ of heat?

the delta H refers to "moles of reaction"

From this point on it's a simple stoichiometry problem.  What do you think the next step is?

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3. Thyroxine, an important hormone that controls the rate of metabolism in the body, can be isolated from the thyroid gland. If 0.455g of thyroxine is dissolved in 10.0 g of benzene, the freezing point of the solution is 5.144 degrees C. Pure benzene freezes at 5.444 degrees C and has a value for the molal freezing point depression constant Kf of 5.12 degree C kg mol^(-1). What is the molecular weight of thyroxine?

(I know to use delta Tf = Kf x mass of solute...but i don't understand how to find the mols of benzene so that i can find its molecular weight.)

molality=moles of solute/kg of solvent

delta Tf=Kfx molality

you should be able to solve it now, you had the formula incorrect.