Test: Vapour Pressure & Raoult's law - Chemistry MCQ

 °R represents the temperature measured on Rankine scale. Typically, boiling point of water is measured as 100 °C on the Celsius scale. The conversion of Celsius to Rankine scale is R = C x 9/5 + 491.67.
Considering boiling point of water as 100 °C, R = 100 x 9/5 + 491.67 = 671.67 °R
°F, °r represent the Fahrenheit and Reaumur scale, respectively.

Deviation is always exhibited by non-solutions. Non- ideal solutions do not obey Raoult’s behavior. This is because ΔH_mix, ΔV_mix ≠ 0. The cause for this is the intermolecular attractions. In this case, molecular attraction between A – B is weaker than that of A – A and B – B. Hence the easier it is to break the bonds and boil faster at a lower temperature than that of either of A and B.

Given,
P⁰eth = 5.95 kPa
P⁰chl = 21.17 kPa
Mole ratio of ethanol ∶ chloroform = 2 ∶ 3
Total number of parts = 2 + 3 = 5
Therefore, mole fraction of ethanol, X_eth = 2/5 = 0.4
Mole fraction of chloroform, X_chl = 3/5 = 0.6
From Raoult’s law, p_A = p⁰A x X_A
P_eth = 5.95 x 0.4 = 2.38 kPa
P_chl = 21.17 x 0.6 = 12.702 kPa
From Dalton’s law, P_total = P_eth + P_chl
P_total = 2.38 + 12.702 = 15.082 kPa.

The vapor pressure of a liquid is independent of the pressure of surroundings. Any solution boils when its vapor pressure equals the surrounding atmospheric pressure. The atmospheric pressure decreases with increase in altitude. Due to this, the vapor pressure equals the atmospheric pressure in a shorter period of time when compared to at the solution being present at sea level. Therefore, the solution will vaporize quickly as the bowling point will reached faster.

More the solute, lower is the vapor pressure of the solvent when compared to that when it is in its purest form. In case of greater amount of non-volatile solute, more solvent can be added to raise the vapor pressure. Therefore, it is always directly proportional to the mole fraction of the solvent.

Boiling, by definition, occurs when the vapor pressure of a liquid equals the surrounding atmospheric pressure. It is a very quick vaporization process which takes place at a constant temperature, referred to as the boiling point.

Dalton’s of partial pressure states that for a non-reactive mixture of gases in a closed vessel, the total pressure of the mixture is the sum of pressures all individual gas components present. If gases A, B and C are present then, total pressure, P_total = P_A + P_B + P_C.

Phenol and aniline both exhibit greater magnitude of hydrogen bonding due to the presence of hydroxyl and amine group, both possessing highly electronegative atoms of oxygen and nitrogen, respectively. The intermolecular hydrogen bonding is far greater than the degree of intramolecular hydrogen bonding which requires more thermal energy to break. Thus, there is an increase in the boiling point of the mixture.

Raoult’s law states that the partial pressure of each component in the solution varies directly with its mole fraction in the solution. It is formulated as p_total = p_A + (p_B – p_A)x_B. From this, it is seen that when a graph is plotted, it gives a linear plot passing through origin. Total vapor pressure can be expressed as mole fraction of one component and that it varies linearly with the latter. However, total vapor solution may decrease or increase with increase in mole fraction of a component.

This phenomenon is known as ‘relative lowering of vapor pressure’ which is a very common colligative property. When a non-volatile solute is added to a solvent, the upper surface of the solvent is covered partially. Hence, the solvent molecules do not bear maximum freedom to escape into the space as vapors. Consequently, there is a decrease in vapor pressure when compared towhat it would have been with pure solvent and no solute at all.