In chromatography, which of the following statements is incorrect for Rf?
(a) Rf value depends on t

Q: Why can't the $R_f$ value be greater than 1?

The solvent front represents the maximum distance the mobile phase has traveled. Since the compounds are carried *by* the solvent, they cannot outpace it. Thus, the compound's distance is always less than or equal to the solvent's distance, making $R_f \le 1$.

Q: Does the $R_f$ value have any units?

No, $R_f$ is a dimensionless quantity. It is the ratio of two distances (e.g., cm/cm), so the units cancel out completely.

Q: Why does changing the solvent change the $R_f$ value?

The solvent acts as the mobile phase. Different solvents have different polarities, which changes how strongly they pull the compound away from the stationary phase. A more compatible solvent will carry the compound further, increasing its $R_f$.

Q: What does a very low $R_f$ value indicate?

A low $R_f$ value (close to 0) indicates that the compound is strongly adsorbed to the stationary phase and has very little affinity for the mobile phase. It barely moved from the origin.

Q: Is the $R_f$ value a universal constant for a specific compound?

Absolutely not! The $R_f$ value is specific to the exact conditions of the experiment, including the type of stationary phase, the exact composition of the mobile phase, and even the temperature.

Question

In chromatography, which of the following statements is incorrect for $R_f$?

Options:

(A) $R_f$ value depends on the type of chromatography

(B) Higher $R_f$ value means higher adsorption

(C) $R_f$ value is dependent on the mobile phase

(D) The value of $R_f$ can not be more than one

Sigma Percentile · Accepted Answer

### The Race on a Silica Track

Imagine you are watching a race, but this is no ordinary race. The track is made of silica gel, and the runners are molecules of different compounds. This is the beautiful world of **Chromatography**, a technique used to separate mixtures based on how much they 'like' the track versus how much they 'like' the wind pushing them forward.

In this analogy, the track is the **stationary phase**, and the wind (the solvent moving up the plate) is the **mobile phase**.

### The Master Equation: Retention Factor ($R_f$)

To measure who is winning the race, chemists use a metric called the Retention Factor, or $R_f$. It is defined mathematically as:

$$R_f = \frac{	ext{Distance travelled by the compound}}{	ext{Distance travelled by the solvent}}$$

Let's analyze the physical constraints of this equation. The solvent is the vehicle carrying the compounds. A compound can never travel faster or further than the solvent itself. Therefore, the numerator can never exceed the denominator.

This leads us to a fundamental rule:

$$R_f \le 1$$

Because of this, we immediately know that option (d) is a correct statement.

### The Tug-of-War: Adsorption vs. Solubility

Now, let's dive into the physics of the separation. As the solvent moves up the plate, the molecules experience a tug-of-war.

The stationary phase tries to hold them back through a process called **adsorption**. If a molecule is highly polar and the silica plate is also polar, they stick together strongly. This is high adsorption.

On the other hand, the mobile phase tries to pull them forward. If a molecule is highly soluble in the solvent, it will race ahead.

### Analyzing the Options

What happens if a compound has **higher adsorption**? It means it is sticking very strongly to the stationary phase. Because it is stuck, it won't travel very far up the plate. Its distance will be small.

Looking back at our master equation, if the distance traveled by the compound is small, the $R_f$ value must also be small!

Therefore, **higher adsorption means a lower $R_f$ value**.

Option (b) claims that a higher $R_f$ value means higher adsorption. This is completely backwards! It defies the very physics of the chromatographic process. Thus, option (b) is the incorrect statement we were looking for.

Finally, what about options (a) and (c)? Since the $R_f$ value depends entirely on the tug-of-war between the stationary and mobile phases, changing either the type of chromatography (the track) or the mobile phase (the wind) will absolutely change the $R_f$ value. So, these statements are perfectly correct.

Animated Solution for Chemistry - Organic Chemistry: In chromatography, which of the following statements is incorrect for $R_{f}$ ?

Select Answer:

Visualized Solution

$C$

$T L C$

$R_{f} = \frac{x}{y}$

$R_{f} \leq 1$

$R_{f} = f (S, M)$

$x \propto \frac{1}{A}$

$R_{f} \propto \frac{1}{A}$

The Sigma Insight: Nomenclature and Characterisation

The Race on a Silica Track

The Master Equation

The Tug-of-War

Analyzing the Options

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Animated Solution for Chemistry - Organic Chemistry: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } In chromatography, which of the following statements is incorrect for Rf​?