Vapour Pressure
When the molecules of a liquid leave the open surface area, they are mixed up with air above the liquid. If the vessel is open these molecules go on leaving the surface area. However, if we close the system the particles of liquid start collecting above the surface. These particles not just collide with the walls of the container, but also with the surface of the liquid as well.
There are chances that these particles are regained by the surface area of the liquid. This process is called condensation. The two processes i.e., evaporation and condensation continue till a stage reaches when the rate of evaporation ends up being equal to the rate of condensation. This is called the state of dynamic equilibrium. So,
“The vapour pressure of a liquid is a pressure exerted by the vapours of the liquid in balance with the liquid at a given temperature level”.
The variety of molecules leaving the surface is simply equal to the number of molecules returning into it at a constant temperature. The molecules which are in the liquid state at any moment might remain in vapour state in the next minute. The magnitude of vapour pressure does not rely on the amount of liquid in the container or the volume of the container. It also does not depend on the surface area of a liquid. The bigger area also provides a larger target for returning the particles, so the rate of condensation also increases.
Factors Affecting Vapour Pressure
There are 4 factors on which vapour pressure depends. They are:
Nature of Liquid
Nature of liquid is described based on its intermolecular forces. That is, as the magnitude of the intermolecular forces increases vapour pressure will reduce.
Effect of Temperature level
As the temperature of the liquid boosts the kinetic energy associated with the liquid likewise increases. And due to this boost in kinetic energy, the escaping tendency of molecule boosts and hence vapour pressure boosts. So, we can conclude that vapour pressure is directly proportional to the temperature level.
Concentration of Solute
The existence of solute in the liquid will decrease the vapour pressure. And this fall in vapour pressure likewise differs with the concentration of solute.
Factors having no effect on vapour pressure are
Humidity
Temperature is the only property that impacts the vapour pressure for a specific quantity of water vapour in the air. Humidity will impact only if all the other variables are constant. So, don’t be puzzled between the effect of temperature and humidity.
Surface Area
Usually, vapour pressure is independent of surface area. Change in surface area will not impact the vapour pressure of a substance.
Vapour Pressure and Temperature Level
The values of vapour pressures of different liquids depend relatively upon the nature of liquids i.e., on the sizes of particles and intermolecular forces, however, the most essential specification which manages the vapour pressure of a liquid is its temperature. At a high temperature, the kinetic energy of particles is enhanced and the ability to leave the surface area increases.
It causes an increase in vapour pressure. The increase of vapour pressure goes on increasing for the very same difference in temperature level from 0 ° C to 100 ° C for water. There is an increase of vapour pressure from 4.579 torrs to 9.209 torrs for change in temperature level from 0 ° C to 10 ° C.
However, the increase is from 527.8 torr to 760 torr when temperature level changes from 90 ° C to 100 ° C. The difference in the strength of intermolecular forces in different liquids is directly related to their vapour pressures at a specific temperature. The stronger the intermolecular forces the lower the vapour pressure.
At 20 ° C isopentane has the greatest vapour pressure, while glycerol has the lowest.
Measurement of Vapour Pressure by Manometric Method
The manometric method is relatively a precise and accurate method. The liquid whose vapour pressure is to be figured out is taken in a flask placed in a thermostat. One end of the tube from the flask is linked to a manometer and the other end is linked to an air pump. The liquid is frozen with the help of a freezing mixture and the space above the liquid is evacuated.
In this way, the air is removed from the surface of the liquid along with the vapours of that liquid. The frozen liquid is then melted to release any entrapped air. The liquid is again frozen and released air to evacuate. This procedure is repeated many times till almost all the air is eliminated.
Now the liquid is warmed in the thermostat to that temperature at which its vapour pressure in the flask is to be identified. The difference in the heights of the columns of-Hg in liquid by the manometric technique the two limbs of the manometer determine the vapour pressure of the liquid. The column of mercury in the manometer facing the vapours of the liquid is depressed.
The other column, which faces the atmospheric pressure, rises. In fact, the pressure on the surface of the liquid in the flask equal to the sum of the air pressure and the vapour pressure of the liquid. For this reason, the column of manometer dealing with the liquid is more depressed than dealing with the atmosphere, and it is provided by the following equation.
P = Pa + Δh
Where,
P = Vapour pressure of the liquid at one atm pressure.
P= Atmospheric pressure.
Δh = Difference in the heights of the mercury levels in the two limbs of the manometer, providing us the vapour pressure of the liquid.
MCQs on Vapour Pressure
- What is the definition of vapour pressure?
- A. Pressure exerted by liquid particles
- B. Pressure exerted by vapours in equilibrium with a liquid
- C. Pressure exerted by the atmosphere
- D. Pressure exerted by solids
Answer: B
- What is the state of dynamic equilibrium in terms of vapour pressure?
- A. Rate of condensation is higher than the rate of evaporation
- B. Rate of evaporation is equal to the rate of condensation
- C. Rate of evaporation is higher than the rate of condensation
- D. Rate of evaporation is constant
Answer: B
- What does the nature of liquid depend on regarding vapour pressure?
- A. Volume
- B. Intermolecular forces
- C. Surface area
- D. Temperature
Answer: B
- How does temperature affect vapour pressure?
- A. Inversely proportional
- B. No effect
- C. Directly proportional
- D. Independent
Answer: C
- What is the effect of solute concentration on vapour pressure?
- A. Increases vapour pressure
- B. No effect
- C. Decreases vapour pressure
- D. Random effect
Answer: C
- Which of the following has no effect on vapour pressure?
- A. Humidity
- B. Surface area
- C. Temperature
- D. Intermolecular forces
Answer: B
- What is the most critical factor controlling vapour pressure in a liquid?
- A. Volume
- B. Nature of liquid
- C. Temperature
- D. Surface area
Answer: C
- Which property specifically affects vapour pressure for a given amount of water vapour in the air?
- A. Temperature
- B. Humidity
- C. Surface area
- D. Volume
Answer: A
- What happens to vapour pressure as the temperature of the liquid increases?
- A. Decreases
- B. Remains constant
- C. Fluctuates
- D. Increases
Answer: D
- According to the text, what does the manometric method measure?
- A. Volume of vapour
- B. Temperature of the liquid
- C. Vapour pressure of a liquid
- D. Humidity
Answer: C
- In the manometric method, what is evacuated from the surface of the liquid along with its vapours?
- A. Air
- B. Liquid particles
- C. Solute
- D. Humidity
Answer: A
- What is the purpose of freezing the liquid in the manometric method?
- A. To decrease vapour pressure
- B. To increase humidity
- C. To release entrapped air
- D. To measure temperature
Answer: C
- How is vapour pressure calculated in the manometric method?
- A. P = Pa – Δh
- B. P = Δh – Pa
- C. P = Pa + Δh
- D. P = Δh + Pa
Answer: C
- What is the relationship between intermolecular forces and vapour pressure?
- A. Directly proportional
- B. Inversely proportional
- C. No relationship
- D. Random relationship
Answer: B
- What happens to vapour pressure when the temperature of a liquid is increased from 0 °C to 100 °C?
- A. Decreases
- B. Increases
- C. Remains constant
- D. Fluctuates
Answer: B
- At 20 °C, which substance has the highest vapour pressure according to the text?
- A. Water
- B. Glycerol
- C. Isopentane
- D. Mercury
Answer: C
- What is the primary factor that controls vapour pressure in different liquids at a specific temperature?
- A. Volume
- B. Surface area
- C. Intermolecular forces
- D. Temperature
Answer: D
10 Problem/Solutions
Problem 1: Problem: A sealed container contains a liquid at a certain temperature. Calculate the vapour pressure if given the concentration of solute in the liquid.
Solution: Use Raoult’s law, which states that the vapour pressure of a solution is the sum of the partial pressures of each component. Vapour pressure can be calculated using the formula:
P_total = P_solvent * X_solvent + P_solute * X_solute
Where P_total is the total vapour pressure, P_solvent and P_solute are the vapour pressures of the solvent and solute, and X_solvent and X_solute are their respective mole fractions.
Problem 2: Problem: A sample of water is in an open container. Calculate the vapour pressure at a given temperature using the provided data on temperature and the vapour pressure of water.
Solution: Use the Antoine equation for water, which relates temperature to vapour pressure. The equation is typically given as:
log10(P) = A – B / (T + C)
Where P is the vapour pressure, T is the temperature in Celsius, and A, B, and C are constants. Plug in the temperature to find the vapour pressure.
Problem 3: Problem: Two liquids with different intermolecular forces are mixed. Calculate the vapour pressure of the resulting solution.
Solution: Use Raoult’s law, considering the mole fractions and vapour pressures of each component. The total vapour pressure can be expressed as:
P_total = X1 * P1 + X2 * P2
Where X1 and X2 are the mole fractions of each component, and P1 and P2 are their respective vapour pressures.
Problem 4: Problem: A substance undergoes phase change from liquid to gas. Determine the vapour pressure at a certain temperature.
Solution: Utilize the Clausius-Clapeyron equation, which relates the vapour pressure to temperature during a phase change:
ln((P2/P1)) = -ΔH_vap / R * (1/T2 – 1/T1)
Where P1 and P2 are the vapour pressures at temperatures T1 and T2, ΔH_vap is the enthalpy of vaporization, and R is the gas constant.
Problem 5: Problem: A liquid in a closed container reaches dynamic equilibrium. Determine the vapour pressure at equilibrium.
Solution: At dynamic equilibrium, the rate of evaporation equals the rate of condensation. Use the ideal gas law to calculate the vapour pressure at equilibrium:
PV = nRT
Where P is the vapour pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature.
Problem 6: Problem: A mixture of gases is in a container. Find the partial pressure of a specific gas based on its mole fraction.
Solution: Use Dalton’s law of partial pressures. The partial pressure (Pi) of a gas in a mixture is given by:
Pi = Xi * P_total
Where Xi is the mole fraction of the specific gas, and P_total is the total pressure of the gas mixture.
Problem 7: Problem: A liquid solution is prepared by mixing two substances. Determine the change in vapour pressure caused by adding a solute.
Solution: Use Raoult’s law to calculate the change in vapour pressure:
ΔP = P_pure solvent – P_solution
Where P_pure solvent is the vapour pressure of the pure solvent, and P_solution is the vapour pressure of the solution.
Problem 8: Problem: A sample of gas is collected over water. Calculate the partial pressure of the dry gas.
Solution: Apply Dalton’s law to determine the partial pressure of the dry gas. The partial pressure (P_dry gas) is given by:
P_dry gas = P_observed – P_water vapour
Where P_observed is the observed pressure, and P_water vapour is the vapour pressure of water at that temperature.
Problem 9: Problem: A liquid evaporates in an open container. Calculate the rate of evaporation when given the vapour pressure and surface area.
Solution: Use the rate of evaporation formula:
Rate of Evaporation = A * P
Where A is the surface area and P is the vapour pressure.
Problem 10: Problem: Two liquids are in a closed container. Determine the total vapour pressure using Raoult’s law.
Solution: Apply Raoult’s law to find the total vapour pressure:
P_total = X1 * P1 + X2 * P2
Where X1 and X2 are the mole fractions of the liquids, and P1 and P2 are their respective vapour pressures.
Summary
In this tutorial on Vapour Pressure, we explored the concept of vapour pressure and its dependence on various factors. The key points covered include:
- Vapour Pressure Definition:
- Vapour pressure is the pressure exerted by vapours in equilibrium with a liquid at a given temperature. It occurs when the rate of evaporation equals the rate of condensation, establishing a dynamic equilibrium.
- Factors Affecting Vapour Pressure:
- Nature of Liquid: Vapour pressure is influenced by intermolecular forces; stronger forces lead to lower vapour pressure.
- Effect of Temperature: As temperature increases, kinetic energy rises, enhancing the escaping tendency of molecules, and subsequently, vapour pressure.
- Concentration of Solute: The presence of solute decreases vapour pressure, and the extent of reduction depends on the solute concentration.
- Factors with No Effect on Vapour Pressure:
- Humidity: Vapour pressure is primarily influenced by temperature, and humidity only plays a role when other variables are constant.
- Surface Area: Vapour pressure is independent of surface area changes.
- Vapour Pressure and Temperature Level:
- Vapour pressures vary among liquids based on their nature and intermolecular forces. Temperature is a crucial factor; higher temperatures increase kinetic energy, leading to higher vapour pressure.
- Measurement of Vapour Pressure by Manometric Method:
- The manometric method is a precise technique involving a flask, a manometer, and an air pump. The liquid’s vapour pressure is determined by freezing, evacuating air and vapours, and measuring the pressure difference using a manometer.
The tutorial provides insights into the fundamental principles of vapour pressure and its measurement, offering a comprehensive understanding of this important physical property.