Thermodynamics: Air and Steam Mixtures

[nightingale]

Homework Statement

A vessel of 0.3m³ capacity contains a mixture of air and steam which is 0.75 dry. If the pressure is 7 bar and temperature is 116.9 degree Celsius, find: Mass of water present, mass of dry saturated vapour, mass of air.
Answer key: 0.102kg, 0.307kg, 1.394kg

Homework Equations

Relative humidity = mv/mg = Pv/Pg (pressure of vapour/ pressure of dry saturated vapour)
humidity ratio = mv/ma
Moist Volume V = total volume/mass of dry air

The Attempt at a Solution

I was able to determine the mass of dry saturated vapour. The moist volume (V) of dry saturated vapour from the steam table is 0.9774m³/kg and hence:

0.9974 = volume/mass of dry saturated vapour
0.9974 = 0.3/mg
mg = 0.3069kg

I have absolutely no idea however, on how to find the other two masses. I don't understand the term 0.75 dry, I don't know how to find the mass of water (is this mass of water vapour?) without relative humidity. I'd appreciate any help. Thank you very much.

 

[maajdl]

What does that mean? :

"... a mixture of air and steam which is 0.75 dry ..."

would that mean that 75% of the water is vapor and 25% is liquid ?
If this is the case, you know that the steam is saturated and you can find its partial pressure Pv.
Then you know also the partial pressure of air: Ptot - Pv = 7 - Pv .

The rest of the problem is then a simple conversion of quantities from partial pressure to masses.

 

[nightingale]

Which means that the air is 100% saturated in order for the saturated vapour (Pg) to be equal to Pv. Am I correct?
Because if from the steam table I would only get Pg, not Pv. But since you said I can find Pv, that means Pg is equal to Pv?

Thank you.

 

[Chestermiller]

Which means that the air is 100% saturated in order for the saturated vapour (Pg) to be equal to Pv. Am I correct?
Because if from the steam table I would only get Pg, not Pv. But since you said I can find Pv, that means Pg is equal to Pv?

Thank you.

Yes.

Chet

 

[nightingale]

Thank you, everyone!