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In this screencast we will look at the idea of humidity.A term specifically reserved for
a water content of air. Or an air water system. So say that you buy a rain forest in a bottle
for your iguana, and you keep the bottle at a temperature of about 90 degrees Fahrenheit,
and the manufacturer told you that this needs to correspond to 80 percent humidity, and
you are Celsius about how much water in the air, in the bottle. So there are a couple
of ways in measuring humidity of a system. First we can do the absolute humidity. The
relative humidity, the mole humidity, or the percentage humidity. So in this case the manufacture
were referring to relative humidity as being at 80 percent. So the equation you would use
to calculate relative humidity is that relative humidity equals the partial pressure of component
i divided by the saturation pressure of component i at a particular temperature times 100 percent,
and so the saturation pressure. We remember is the pressure where vapor will begin to
condense. So at 100 percent relative humidity water will just begin to come out of the air.
So for our system. We know that we have 80 percent relative humidity. We don't yet know
what the partial pressure yet is for component i, or Pi sat, at our temperature. So we need
to look up saturation pressure of water. From a table, and we convert or temperature into
Celsius. We see that we need to find the saturation pressure at 32.2 degree Celsius. Looking at
our chart we find 32.2 and we see that the saturation pressure is 36.068 mmHg. So now
that we have the saturation pressure. We can use this to calculate what the partial pressure
of component i is in our system. So plugging in our numbers and solving for the partial
pressure of component i, which is the water in the system. We find that the partial pressure
is 28.854 mmHg. So now we have solved for the partial pressure of the water in our system.
So lets use this number to calculate 1 other property used form of humidity. The absolute
humidity. So the absolute humidity of a system is defined as the mass of vapor in the system
divided by the mass of dry gas. So we can write this mathematically as Pi the partial
pressure of component i times the molecular weight of component i, divided by the total
pressure minus the partial pressure of component i, times the molecular weight of the dry air.
So if we plug in our numbers to this equation we remember from above that our partial pressure
of the water in the system is 28.854 mmHg the molecular weight of water is 18 g/mol.
The atmospheric pressure of the system. The total pressure is 760 mmHg. We subtract from
that the partial pressure due to the water vapor. Times the average molecular weight
of air, which is 29 g/mol, and this is going to give us 0.024 grams of water per gram of
air, and these two calculations are the two most common types of humidity we can calculate
for a system.