Question

The elevation in boiling point method is used for the determination of molecular masses of (a) non-volatile and soluble solute (b) non-volatile and insoluble solute (c) volatile and soluble solute (d) volatile and insoluble solute

Short answer

The elevation in boiling point method is used for the determination of molecular masses of non-volatile and soluble solutes.

Step-by-step solution

Understanding Boiling Point Elevation

Boiling point elevation is a colligative property that states when a non-volatile solute is added to a solvent, the boiling point of the solvent increases. This is because the presence of solute particles disrupts the ability of solvent molecules to escape into the gas phase, requiring a higher temperature to boil.

Analyze Solubility

The method for determining molecular masses of substances using boiling point elevation requires that the solute be soluble since it has to uniformly dissolve within the solvent to affect its boiling point.

Consider Volatility

The solute must be non-volatile. Indeed, if a solute is volatile, it will partake in the gas phase, contributing to the system's vapor pressure itself and complicating the determination of the molecular mass from the boiling point elevation.

Choice between Non-volatile and Soluble Solutes

Based on the above considerations, we can deduce that boiling point elevation can be used to determine the molecular masses of only non-volatile and soluble solute because this ensures that the solute affects the boiling point without complicating factors such as solute vaporization.

Key concepts

Colligative Properties

Colligative properties are those that depend on the ratio of solute to solvent particles, rather than the identity of the solute itself. This means they are properties of solutions that can change when you add more solute particles into the solvent, regardless of the type of particles added. Examples of colligative properties include vapor pressure lowering, freezing point depression, osmotic pressure, and, pertinent to our topic, boiling point elevation.

The principle behind boiling point elevation is relatively simple: adding a non-volatile solute to a solvent reduces the number of solvent molecules able to escape as gas at the liquid's surface. Because fewer solvent particles can escape into the vapor phase, the vapor pressure of the solvent decreases. To achieve boiling, the system must be heated to a higher temperature to make up for this decrease in vapor pressure, allowing it to equal the atmospheric pressure. Consequently, the solution’s boiling point is higher than that of the pure solvent. It's essential to note that boiling point elevation is directly proportional to the molality (moles of solute per kilogram of solvent) of the solution.

Quantitative Analysis with Boiling Point Elevation

Using the formula \(\Delta T_b = K_b \cdot m\), where \(\Delta T_b\) is the boiling point elevation, \(K_b\) is the ebullioscopic constant of the solvent, and \(m\) is the molality of the solution, we can perform a quantitative analysis of the solution and determine how much the boiling point is elevated due to the dissolved solute.

Molecular Mass Determination

Determining the molecular mass of a solute using boiling point elevation is a practical application of colligative properties. The process is often employed for substances that are non-volatile and have sufficient solubility in a given solvent.

When a known mass of a solute is dissolved in a known mass of a solvent, causing the solution to boil at a temperature higher than the solvent's normal boiling point, the elevation in temperature is used to calculate the molecular mass. The equation \( m = \frac{\text{moles of solute}}{\text{kilograms of solvent}} \) allows us to find the molality. Considering the boiling point elevation equation \( \Delta T_b = K_b \cdot m \) and rearranging it, the molecular mass (M) of the solute can be determined:

\( M = \frac{\text{mass of solute} \cdot K_b}{\Delta T_b \cdot \text{mass of solvent}} \)

Thus, by accurately measuring the boiling point elevation and knowing the mass of the solute and solvent and the ebullioscopic constant, the molecular mass of a solute can be accurately deduced. It is a technique particularly useful for large, non-volatile molecules such as polymers and proteins.

Non-Volatile Solute

The characteristic trait of a non-volatile solute is that it does not readily evaporate or sublimate under existing conditions, unlike a volatile substance which easily transitions into the vapor phase. In the context of boiling point elevation and colligative properties, a non-volatile solute is ideal because it does not affect the vapor pressure of the solvent by introducing additional particles into the vapor phase.

A non-volatile solute added to a solvent creates a solution with a higher boiling point than the pure solvent, but crucially, the non-volatile nature of the solute ensures that the boiling point elevation observed is solely due to the solute's effect on the solvent's vapor pressure. This simplifies the calculation of molecular mass since the solute does not contribute to the system's vapor pressure, thus providing reliable data.

Why Non-Volatile Solutes Matter

For an accurate determination of molecular mass using boiling point elevation, it is critical to choose a solute that won't vaporize at the increased boiling point of the solution. The presence of a volatile solute would complicate the calculations, as it would contribute its own vapor pressure, likely resulting in erroneous molecular mass determinations. Non-volatile solutes ensure the precision and reliability of this method for molecular mass determination.