# How to Calculate Vapor Pressure?

##### 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 vapor in equilibrium with its non-vapor phases. The vapor pressure of a liquid is usually much lower than the surrounding atmospheric pressure, so when a liquid evaporates, it cools down. There are several ways to calculate vapor pressure.

The most common method is the Antoine Equation. The Antoine Equation was developed in 1888 by French chemist Marcel-Eugene Antoine and can be used to estimate the vapor pressure of liquids over a wide range of temperatures. To use the Antoine Equation, you need to know the A, B, and C coefficients for the substance you are trying to calculate the vapor pressure for.

These coefficients can be found in tables or online (see Resources). Once you have these coefficients, plug them into the equation: ln(P) = A – (B/(T+C))

P = Vapor Pressure (mmHg) A = A Coefficient from Table/Online Resource B = B Coefficient from Table/Online Resource

T = Temperature (Kelvin)

• Look up the vapor pressure curve for the substance of interest
• This can be found in a variety of reference books or online
• Find the temperature of interest on the x-axis of the curve
• Starting from the bottom left of the curve, draw a line straight up until it intersects the curve
• Draw a line horizontally to the y-axis to determine the vapor pressure at that temperature

## How Do You Calculate Vapor Pressure in Raoult’S Law?

In order to calculate vapor pressure in Raoult’s Law, you must first determine the mole fraction of the solvent. This can be done by dividing the number of moles of solvent by the total number of moles in the solution. Once you have determined the mole fraction, you must then multiply it by the vapor pressure of pure solvent.

This will give you the vapor pressure of the solution.

## 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 atmospheric pressure. This is the pressure of the air around you and is typically given in units of mmHg or inches of mercury.

2) The boiling point of the liquid. This is the temperature at which the liquid boils and turns into a gas. It is typically given in units of degrees Celsius or Fahrenheit.

3) The vapor pressure of water at that temperature. Water has a unique property in that its vapor pressure increases as its temperature increases. Therefore, you must look up the vapor pressure of water at your particular boiling point temperature using a reference like this one: http://www.engineeringtoolbox.com/water-vapor-pressure-d_595.html .

Once you have these three pieces of information, you can plug them into the following equation to calculate vapor pressure: Vapor Pressure = Atmospheric Pressure – (Vapor Pressure of Water x Boiling Point Temperature) For example, let’s say we want to calculate the vapor pressure at sea level (atmospheric pressure = 760 mmHg) when water boils at 100 degrees Celsius (212 degrees Fahrenheit).

We would first look up the vapor pressure of water at 100 degrees Celsius and find that it is 1 atmosphere (atm). We would then plug these values into our equation and solve for Vapor Pressure: Vapor Pressure = 760 mmHg – (1 atm x 100 degrees Celsius)

## How Do You Solve Vapor Pressure Problems?

In order to solve vapor pressure problems, you need to have a firm understanding of the concept of vapor pressure. Vapor pressure is the pressure exerted by a gas in equilibrium with its liquid or solid phase. The higher the vapor pressure of a given substance, the more likely it is to be in the gas phase at any given temperature.

One way to think about vapor pressure is in terms of evaporation. When a liquid is heated, its molecules gain energy and begin to move faster. Some of these molecules will have enough energy to escape from the surface of the liquid and enter into the surrounding air.

This process is called evaporation, and it results in a lowering of the liquid’s temperature. The reverse process, when molecules from the air collide with and fall back into theliquid, is called condensation. The rate at which evaporation occurs depends on several factors, including the vapor pressure of the liquid (higher vapor pressures mean higher rates of evaporation), as well as temperature (higher temperatures mean faster molecular motion and thus higher rates of evaporation).

If we imagine a closed container filled with a liquid, then over time as Evaporatation occurs ,the concentration Ofgas molecules abovethe surfaceof th eliquid will increase .At some point , th erateof evaporat ionwill equalth erate Ofcondensat ionand th e systemwill reacha stat eOfequilibri um . Th e pres sureexert edbythesethe gasmoleculesin equilibri umwiththesolidorliquidphaseis whatwe callvaporpressure .

Vapor Pressure = Concentration Of Gas Molecules * R * Temperature R= 8.3145 J/K*mol Temperature must be in Kelvin

Concentration can be found by using Ideal Gas Law

## How to Calculate Vapor Pressure from Boiling Point

If you know the boiling point of a liquid, you can calculate its vapor pressure. The vapor pressure is the pressure that the vapors of a liquid exert on the walls of their container. It is also a measure of the strength of the forces between molecules in a liquid.

To calculate vapor pressure, you need to know two things: the boiling point of the liquid and the atmospheric pressure. The atmospheric pressure is the weight of air pushing down on an area. It is measured in units called atmospheres (atm).

1 atm = 14.7 psi The boiling point is the temperature at which a liquid changes into a gas. The higher the vapor pressure, the lower the boiling point.

For example, water boils at 100°C (212°F) at sea level, but only at 93°C (199°F) at high altitude where there is less atmospheric pressure. Here’s how to calculate vapor pressure: Vapor Pressure = Atmospheric Pressure * e^(-Boiling Point/Temperature)

where: Vapor Pressure = absolutepressureofthevaporabovealiquid e = baseofnaturallogarithms(2.71828…) Boiling Point = temperatuareatthewaterboils(100°Catsealevel) Temperature= absolutezeroinKelvinorRankine(0K or 0R) Absolute zero in Kelvin or Rankine= -459.67°F AtmosphericPressure= standardatmosphericpressure(14.7psi or 2992mmHg or 1013mbar) Standardatmosphericpressure= 1atmosphere(101325Paor1013mbaror1462mmHgord2992inchesHgor760millimetersofmercury[torr]) The formula for calculating vapor pressure can be simplified if you know the value for “e” and plug it in:

## How to Calculate 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 pressure at which water vapor is in equilibrium with its liquid or solid phase. The higher the temperature, the higher the vapor pressure.

There are several ways to calculate the vapor pressure of water. One way is to use the Antoine equation: log10P = A – (B/(C+T))

where P is Vapor Pressure (in mmHg), T is Temperature (in °C), A, B, and C are coefficients that vary depending on the range of temperatures being used. For example, for water at 25°C, A = 8.07131, B = 1730.63, and C = 233.426. To find P at 25°C using this equation would be: log10P = 8.07131 – (1730.63/(233.426+25)).

This gives a result of P= 1 atmosphere or 760 mmHg, which we know to be true from looking at a phase diagram for water (the triple point for water is 0°C and 1 atmosphere). Another way to calculate vapor pressure uses what’s called the Clausius–Clapeyron equation: ????????=(????/????)(1/[(−∂????????)/∂????])exp[(−∂S????)/(R∂T)] Where Eis change in enthalpy per mole of evaporated molecules (kJ mol-1) Rgas constant J mol-1 K-1 ∂Hvapchange in enthalpy with change in temperature at constant volume kJ mol-1 K-1 ∂Svvchange in entropy with change in temperature at constant volume J mol-1 K-1 Ttemperature Kelvin For example purposes lets say we want to find out what the vapor pressure would be if we increased the temperature by 10K . We can set up our equation as follows: PV2=(EH2o/R)(1/[(-46000)/(85600)])exp[(-188000)/((85600)*29815)] This gives us an answer of 2200mm Hg .

## How to Calculate Vapor Pressure at a Given Temperature

Vapor pressure is the pressure of a gas above a liquid. The vapor pressure of a liquid increases as the temperature increases. The formula for vapor pressure is:

P = RT/V where P is the vapor pressure, R is the universal gas constant, T is the absolute temperature, and V is the molar volume. Molar volume can be calculated using the Ideal Gas Law:

PV = nRT where n is the number of moles and R is again the universal gas constant. Therefore, we can solve for V in terms of n and T:

V = nRT/P Plugging this into our original equation for P, we get: P = (nRT/P)T/V

## Vapor Pressure Lowering Formula

In order to find the vapor pressure lowering of a given substance, one must use the vapor pressure lowering formula. This formula takes into account the presence of another substance with which the given substance is in contact. The vapor pressure lowering formula is as follows:

P1 = P0 – X * (P0 – P2) Where: X = mole fraction of solute

P1 = Vapor Pressure of Solution P0 = Vapor Pressure of Solute

## Conclusion

Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. The vapor pressure of a liquid is usually much lower than the atmospheric pressure, so most liquids boil at temperatures well below their flash points. To calculate the vapor pressure of a liquid, you’ll need to know the boiling point of the liquid and the atmospheric pressure.

The higher the boiling point, the higher the vapor pressure. The atmospheric pressure will also affect the vapor pressure – if it’s high, the vapor pressure will be high, and if it’s low, the vapor pressure will be low. Once you have those two pieces of information, you can use them to calculate the vapor pressure using this formula:

P = 760 mmHg * e^( – ( bp – T ) / T ) where P is Vapor Pressure, bp is Boiling Point, T is Temperature in Kelvin and mmHg is millimeters of mercury.

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.