Vapour Pressure of Pure Water

Oct 13, 2025 by magicalceylontours.com 74 views

Introduction

Vapour pressure is a measure of the tendency of a liquid to evaporate. It is an important property of a solution, as it can affect the boiling point and other physical properties. In this article, we will calculate the vapour pressure of a solution containing urea dissolved in water at a given temperature.

Vapour Pressure of Pure Water

The vapour pressure of pure water at 295 K is given as 8.5 mm of Hg. This value is a standard reference point for calculating the vapour pressure of solutions.

Raoult's Law

Raoult's Law states that the vapour pressure of a solution is directly proportional to the mole fraction of the solvent. Mathematically, it can be expressed as:

P_solution = P_solvent × X_solvent

where P_solution is the vapour pressure of the solution, P_solvent is the vapour pressure of the pure solvent, and X_solvent is the mole fraction of the solvent.

Mole Fraction of Water

To calculate the mole fraction of water, we need to know the number of moles of water and urea. The molar mass of water is 18 g/mol, and the molar mass of urea is 60 g/mol.

Number of moles of water = mass of water / molar mass of water = 400 g / 18 g/mol = 22.22 mol

Number of moles of urea = mass of urea / molar mass of urea = 25 g / 60 g/mol = 0.4167 mol

Total number of moles = number of moles of water + number of moles of urea = 22.22 mol + 0.4167 mol = 22.6367 mol

Mole fraction of water = number of moles of water / total number of moles = 22.22 mol / 22.6367 mol = 0.9815

Vapour Pressure of the Solution

Now that we have the mole fraction of water, we can calculate the vapour pressure of the solution using Raoult's Law.

P_solution = P_solvent × X_solvent = 8.5 mm of Hg × 0.9815 = 8.36 mm of Hg

Conclusion

In this article, we calculated the vapour pressure of a solution containing urea dissolved in water at a given temperature. We used Raoult's Law to calculate the vapour pressure of the solution, and found that it is 8.36 mm of Hg.

Limitations of the Calculation

It is worth noting that this calculation assumes that the solution is ideal, meaning that the interactions between the solvent and solute molecules are the same as those between the solvent molecules. In reality, the solution may not be ideal, and the vapour pressure may be affected by other factors such as the concentration of the solute and the temperature of the solution.

Future Work

In future work, it would be interesting to experimentally measure the vapour pressure of the solution and compare it to the calculated value. This would help to validate the calculation and provide a more accurate understanding of the vapour pressure of the solution.

References

Raoult, F. (1887). "La loi générale des pressions de vapeur des dissolutions." Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences, 104, 1253-1256.
Atkins, P. W. (1998). Physical Chemistry. Oxford University Press.

Appendix

The following table summarizes the calculations performed in this article.

Quantity	Value
Molar mass of water	18 g/mol
Molar mass of urea	60 g/mol
Mass of water	400 g
Mass of urea	25 g
Number of moles of water	22.22 mol
Number of moles of urea	0.4167 mol
Total number of moles	22.6367 mol
Mole fraction of water	0.9815
Vapour pressure of pure water	8.5 mm of Hg
Vapour pressure of the solution	8.36 mm of Hg

Introduction

In our previous article, we calculated the vapour pressure of a solution containing urea dissolved in water at a given temperature. In this article, we will answer some frequently asked questions related to the topic.

Q: What is vapour pressure?

A: Vapour pressure is a measure of the tendency of a liquid to evaporate. It is an important property of a solution, as it can affect the boiling point and other physical properties.

Q: What is Raoult's Law?

A: Raoult's Law states that the vapour pressure of a solution is directly proportional to the mole fraction of the solvent. Mathematically, it can be expressed as:

P_solution = P_solvent × X_solvent

where P_solution is the vapour pressure of the solution, P_solvent is the vapour pressure of the pure solvent, and X_solvent is the mole fraction of the solvent.

Q: How do you calculate the mole fraction of a solvent?

A: To calculate the mole fraction of a solvent, you need to know the number of moles of the solvent and the total number of moles of the solution. The mole fraction of the solvent is then calculated as:

X_solvent = number of moles of solvent / total number of moles

Q: What is the effect of temperature on vapour pressure?

A: The vapour pressure of a solution increases with temperature. This is because higher temperatures provide more energy for the molecules to escape the surface of the solution.

Q: Can you give an example of how to calculate the vapour pressure of a solution?

A: Let's say we have a solution containing 25 g of urea dissolved in 400 g of water at 295 K. The molar mass of water is 18 g/mol, and the molar mass of urea is 60 g/mol. We can calculate the vapour pressure of the solution as follows:

Number of moles of water = mass of water / molar mass of water = 400 g / 18 g/mol = 22.22 mol

Number of moles of urea = mass of urea / molar mass of urea = 25 g / 60 g/mol = 0.4167 mol

Total number of moles = number of moles of water + number of moles of urea = 22.22 mol + 0.4167 mol = 22.6367 mol

Mole fraction of water = number of moles of water / total number of moles = 22.22 mol / 22.6367 mol = 0.9815

Vapour pressure of pure water = 8.5 mm of Hg

Vapour pressure of the solution = P_solvent × X_solvent = 8.5 mm of Hg × 0.9815 = 8.36 mm of Hg

Q: What are some common applications of vapour pressure?

A: Vapour pressure is an important property of solutions, and it has many practical applications. Some common applications include:

Boiling point elevation: Vapour pressure is used to calculate the boiling point of a solution.
Freezing point depression: Vapour pressure is used to calculate the freezing point of a solution.
Distillation: Vapour pressure is used to separate mixtures of liquids based on their boiling points.
Solvent selection: Vapour pressure is used to select the best solvent for a particular application.

Q: What are some common mistakes to avoid when calculating vapour pressure?

A: Some common mistakes to avoid when calculating vapour pressure include:

Not accounting for the mole fraction of the solvent.
Not using the correct values for the vapour pressure of the pure solvent and the mole fraction of the solvent.
Not considering the effect of temperature on vapour pressure.

Conclusion

In this article, we answered some frequently asked questions related to the vapour pressure of water and urea solution. We hope that this article has provided a better understanding of the topic and has helped to clarify any confusion.