# How to Solve Vapor Pressure Problems? ##### Joseph Hebert
Owner at - HVAC Buster

Joseph is an HVAC technician and a hobbyist blogger. He’s been working as an HVAC technician for almost 13 years, and he started blogging just...Read more

Vapor pressure is the pressure of a gas above its critical temperature. The vapor pressure of a substance increases with increasing temperature. When the vapor pressure of a substance reaches the atmospheric pressure, the substance will boil.

The boiling point of a substance is the temperature at which the vapor pressure equals the atmospheric pressure. The boiling point of water is 100°C (212°F). The ideal gas law can be used to solve for the vapor pressure of a given substance.

This law states that PV = nRT, where P is thepressure, V isthe volume, nis thenumber of moles, R isthe universal gas constant, andTis themeasured in Kelvin. To solve for P, rearrange this equation to read P = (nRT)/V. With this equation, you can plug in known values to find an unknown value forP.

• Read the problem and identify the known and unknown quantities
• Use the ideal gas law to relate the known quantity (usually temperature) to the unknown vapor pressure
• Substitute your values into the equation and solve for the vapor pressure

## How to Find Vapor Pressure from Temperature

Vapor pressure is the measure of a liquid’s ability to turn into vapor. The higher the vapor pressure, the more likely the liquid is to turn into vapor. Vapor pressure is affected by temperature – as temperature increases, so does vapor pressure.

To find the vapor pressure at a given temperature, you’ll need to consult a chart or table that lists vapor pressures at different temperatures. These can be found in many Chemistry and Physics textbooks. Once you have located the correct chart or table, locate the temperature on one axis and read across to find the corresponding vapor pressure on the other axis.

Keep in mind that these charts and tables are only approximations – real world values may vary slightly due to factors such as impurities in the sample being tested.

## Vapor Pressure Lowering Example Problems With Solutions

Vapor pressure is the pressure of a gas in equilibrium with its non-vapor phase. The vapor pressure of a liquid is the equilibrium pressure from the gas phase in contact with the liquid phase. The equilibrium vapor pressure is determined by the boiling point temperature.

The higher the boiling point, the higher the vapor pressure will be. When a solute is added to a solvent, it lowers the vapor pressure of that solvent. This lowering of vapor pressure happens because some molecules of solute are now occupying space where solvent molecules could be present and exerting their Vapor Pressure.

When this occurs, we say that the Vapor Pressure has been lowered by an amount equal to ΔP = P° – P where P° is pure solvent vapor pressure and P is new total mixture vapor pressures . This means that for any given mole fraction or weight percent composition , we can calculate how much lower the Vapor Pressure will be for our solution. Below are three worked out examples showing how to do just this!

Mole Fraction: X acetone = 0.5 X water = 0.5

P° acetone= 2260 mmHg (at 20 degrees C) P° water = 1740 mmHg (at 20 degrees C) example 1: what is Xacetone if Ptotal = 2200mmHg ?

work : PTOTAL= XPACETONE * PPACETONE + XPWATER * PPWATER PTOTAL= .5*2260+.5*1740 solve for Xacetone: .5= Xacetone(2260/.4400) answer: Xacetone= .45 or 45% example 2: what is PTOTAL if XPACETONE= .4 ?

## How to Find Vapor Pressure of a Solution

When it comes to finding the vapor pressure of a solution, there are a few different methods that can be used. One method is to use a solvent with known properties and measure the amount of change in volume that occurs when the solution is heated. Another common method is to use an apparatus called a still pot which essentially boils off the liquid until only the vapors remain.

The temperature and pressure at which this happens can then be used to calculate the vapor pressure. No matter which method you choose, it’s important to have accurate measurements in order to get an accurate result. For example, when using the still pot method, it’s crucial to know the exact starting weight of your solution as well as the final weight of just the vapors.

Once you have these numbers, you can plug them into a formula along with the boiling point temperature of your solvent in order to calculate vapor pressure. With any scientific calculation, accuracy is key so be sure to take your time and double check your work before coming to any conclusions.

## How to Find Vapor Pressure of Water

Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. The vapor pressure of water is the partial pressure of water vapor in air, and it increases as the temperature increases. The higher the vapor pressure, the more likely it is for water to evaporate.

To find the vapor pressure of water, you’ll need to use a thermometer and a barometer. First, take the temperature of the water using the thermometer. Then, use the barometer to measure atmospheric pressure.

Finally, subtract atmospheric pressure from Vapor Pressure (in mmHg) to get your answer. For example: let’s say that you want to find the vapor pressure of water at 20 degrees Celsius. First, you would measure the temperature and find that it is 20 degrees Celsius.

Then, you would use your barometer to measure atmospheric pressure and find that it is 760 mmHg. To calculate the vaporpressureofwaterat20degreesCelsiusyouwouldsubtract760mmHgfromVaporPressure(inmmHg), which would give you 740 mmHg.

## Vapor Pressure Lowering Formula

When a substance is added to a liquid, the vapor pressure of the liquid decreases. This is because the molecules of the substance being added take up space in the liquid, and this reduces the amount of space that molecules have to move around. This reduced space results in fewer molecules escaping from the surface of the liquid into the gas phase, and thus, a lower vapor pressure.

The vapor pressure lowering (VPL) formula can be used to calculate how much a given substance will reduce the vapor pressure of a liquid. The VPL formula is: where P is the original vapor pressure of the pureliquid, P’ is the new vapor pressure after addition ofthe solute, x is molesofsolute/total molesofliquid,and R isthe universalgasconstant(0.08206L·atm/mol·K).

This formula can be rearranged to solve for x, which gives us: Thus, we can use this equation to determine how much solute must be added to a given volume of liquid in order to achieve a desired decrease in vapor pressure. For example, let’s say we have 1 Lof water at 100 kPa and we wantto add enough sodium chloride (NaCl)to decrease itsvaporpressureto 50 kPa.

We would first calculate x using our rearranged VPL equation: Now that we know we need 0.1 mol NaCl per literofwater(or100 g NaCl per literofwater),we can simply add this amountofsodiumchlorideinto our 1 L bottleofthewaterand stir until it has dissolved completely. After doing so, we would measurethevaporpressureofthewateragain and verifythatitisindeed50kPa.

## What is the Vapor Pressure of the Solution at 25?

Assuming you are referring to a vapor pressure of 25 mmHg, this is considered a relatively low vapor pressure. To achieve this level of vapor pressure, the solution would need to be cooled to a temperature below the freezing point of water.

## How Do You Calculate Vapor Pressure from Boiling Point?

To calculate the vapor pressure from boiling point, you need to know the following: 1) The boiling point of the liquid 2) The atmospheric pressure

3) The latent heat of vaporization of the liquid 4) The molar mass of the liquid 5) The universal gas constant.

Here is the equation you will use: VP = X * P* e^((-latent heat of vaporization)/(universal gas constant * molar mass)) Latent heat of vaporization can be found in a variety of ways, but a common one is to look it up in a steam tables book.

Molar mass can also be looked up, or calculated if the molecular formula for the substance is known. Universal gas constant should be given to you in most cases, but if not it is equal to 8.314510 J/K•mol. Finally, X is a correction factor that compensates for non-ideal conditions.

It is equal to 1 at standard temperature and pressure (STP), but will be different at other conditions. You can find values for X in steam tables books as well, or by using an online calculator like this one: http://www.engineeringtoolbox.com/vapor-pressure-d_212.html . Just enter in your desired temperatures and pressures and it will give you a value for X to use in your equation.

## Conclusion

If you’re having trouble solving vapor pressure problems, there are a few things you can try. First, make sure that you understand the definition of vapor pressure. Once you’ve done that, try using the Ideal Gas Law to solve the problem.

If that doesn’t work, you can also try using the Clausius-Clapeyron Equation. Finally, if all else fails, consult a textbook or ask a professor for help. Joseph is an HVAC technician and a hobbyist blogger. He’s been working as an HVAC technician for almost 13 years, and he started blogging just a couple of years ago. Joseph loves to talk about HVAC devices, their uses, maintenance, installation, fixing, and different problems people face with their HVAC devices. He created Hvacbuster to share his knowledge and decade of experiences with people who don’t have any prior knowledge about these devices.