Why isn't the temperature of this vapor T_sat?

[zachdr1]

A rigid tank with a volume of 2.00m3 contains 5.77 kg of saturated liquid-vapor mixture of water at 75 deg C. Now the water is slowly heated. Determine the temperature at which the liquid in the tank is completely vaporized.

Why isn't the temperature just 75C? If it is already a mixture in beginning, then shouldn't it's current temperature be the saturation temperature?

 

[Bystander]

How many phases are present?

 

[zachdr1]

How many phases are present?

It's a mixture of gas and fluid in the initial state, then in the final state it's just a gas

 

[Bystander]

final state it's just a gas

How many phases?

 

[zachdr1]

How many phases?

Wouldn't it just be two? A mixture in the initial and a saturated vapor in the final. It doesn't say anything about the vapor being superheated

 

[Bystander]

Does it have to be "superheated" to be a single phase?

 

[zachdr1]

Does it have to be "superheated" to be a single phase?

its a single phase in the final state but unless more heat is added after the water is evaporated then I would think the temperature would still be the saturation temperature since its a saturated gas

 

[Bystander]

How many moles of water?

 

[zachdr1]

How many moles of water?

I'm not sure, I could look up the molar mass and multiply it by the mass in grams but I'm not sure how using mols would help

 

[zachdr1]

I found the answer by using a saturated water table..it's something like 115 C, I just don't understand why its not 75 C

 

[Bystander]

volume of 2.00m3 contains 5.77 kg

How many moles?

 

[zachdr1]

How many moles?

1.0398e+05 mol

 

[Bystander]

How many moles in one kg of water?

 

[zachdr1]

How many moles in one kg of water?

Why will finding the number of moles help?

 

[Bystander]

You have a fixed volume. It limits the amount of substance.

 

[Chestermiller]

As long as the mixture in the tank is 2 phases, all points along the saturation curve are valid combinations of temperature and pressure. The temperature can rise above 75C because, as you add heat, the pressure can rise, since the mixture is confined to constant volume. Only if the mixture were held at constant pressure would the temperature have to remain at 75 C.

 

[zachdr1]

As long as the mixture in the tank is 2 phases, all points along the saturation curve are valid combinations of temperature and pressure. The temperature can rise above 75C because, as you add heat, the pressure can rise, since the mixture is confined to constant volume. Only if the mixture were held at constant pressure would the temperature have to remain at 75 C.

Ohh okay that makes sense. Thank you!