# I Why isn't the temperature of this vapor T_sat?

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1. Sep 13, 2016

### 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?

2. Sep 13, 2016

### Bystander

How many phases are present?

3. Sep 13, 2016

### zachdr1

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

4. Sep 13, 2016

### Bystander

How many phases?

5. Sep 13, 2016

### zachdr1

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

6. Sep 13, 2016

### Bystander

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

7. Sep 13, 2016

### zachdr1

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

8. Sep 13, 2016

### Bystander

How many moles of water?

9. Sep 13, 2016

### zachdr1

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

10. Sep 13, 2016

### 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

11. Sep 13, 2016

### Bystander

How many moles?

12. Sep 13, 2016

### zachdr1

1.0398e+05 mol

13. Sep 13, 2016

### Bystander

How many moles in one kg of water?

14. Sep 13, 2016

### zachdr1

Why will finding the number of moles help?

15. Sep 13, 2016

### Bystander

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

16. Sep 13, 2016

### Staff: Mentor

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.

17. Sep 14, 2016

### zachdr1

Ohh okay that makes sense. Thank you!