CUET UG Chemistry Booster Test – 3  Conductance of Electrolytic Solutions

According to NCERT, the resistance of an electrolytic solution is measured using a Wheatstone bridge arrangement. This method allows accurate determination of unknown resistance while minimizing errors. For conductivity measurements, the conductivity cell must first be calibrated. This calibration is carried out using a potassium chloride (KCl) solution whose conductivity is known accurately. By measuring the resistance of the KCl solution, the cell constant can be determined. Statement 1 is incorrect because resistance is inversely proportional to the area of cross-section and directly proportional to the length of the conductor. Statement 2 is correct because Wheatstone bridge is used for resistance measurements. Statement 3 is correct because standard KCl solution is used to determine the cell constant. Statement 4 is incorrect because resistance varies with temperature. Therefore, only Statements 2 and 3 are correct.

For an electrolytic solution, conductance is given by: where κ is conductivity, A is the area of cross-section and l is the distance between electrodes. Initially: When the distance between electrodes is doubled: When the area is halved: Substituting into the conductance expression: Thus, the conductance becomes one-fourth of its original value. Therefore, conductance decreases by a factor of four.

Conductivity (κ) is related to resistivity (ρ) by: Given: Therefore: To convert S cm⁻¹ into S m⁻¹: Thus: Hence, both values represent the same conductivity expressed in different units. Therefore, Options A and B are both correct, making Option D the correct answer.

The cell constant is an important parameter used in conductivity measurements. According to NCERT: where l is the distance between the electrodes and A is the area of cross-section of the electrodes. Since length has units of metre (m) and area has units of square metre (m²): Similarly, when centimetres are used: Therefore, the cell constant has units of m⁻¹ or cm⁻¹. It depends only on the geometry of the conductivity cell and not on the nature or concentration of the solution being studied.

The conductivity of an electrolytic solution is related to resistance, resistivity and cell geometry. The ratio l/A is known as the cell constant (G*). When l/A equals unity, conductivity numerically becomes equal to conductance. Therefore, l/A = 1 m⁻¹ corresponds to ideal cell parameters. The relation l/A = G* is the standard equation defining the cell constant. Conductivity is defined as the reciprocal of resistivity, giving κ = 1/ρ. Furthermore, since conductance G = 1/R and conductivity is related to conductance through the cell constant, the expression κ = G*(1/R) is used to calculate conductivity. Thus the correct matching is: 1-d, 2-c, 3-b, 4-a.

Conductivity values must be expressed in a common unit before comparison. According to NCERT, conductivity is commonly expressed in S m⁻¹. Converting all values: 1. 10 S m⁻¹ = 10 S m⁻¹ 2. 1 S cm⁻¹ = 100 S m⁻¹ 3. 0.1 S cm⁻¹ = 10 S m⁻¹ 4. 0.01 S m⁻¹ = 0.01 S m⁻¹ After conversion: 2 = 100 S m⁻¹ 3 = 10 S m⁻¹ 1 = 10 S m⁻¹ 4 = 0.01 S m⁻¹ Thus, the largest value is 2, followed by 3 and 1, and finally 4. Conductivity comparisons are important because different units are frequently used in electrochemistry. Proper unit conversion prevents errors in ranking conductivity values. Therefore, the decreasing order is 2, 3, 1, 4.

Metallic or electronic conductance occurs because free electrons move through the metallic lattice. According to NCERT, electrons enter the conductor from one end and leave through the other end while carrying electrical charge. During this process, the chemical composition of the metallic conductor remains unchanged. Unlike electrolytic conductance, metallic conductance does not involve chemical reactions or movement of ions. Therefore, no alteration occurs in the composition of the conductor. This is an important distinction between electronic conductance and ionic conductance. Temperature may vary during conduction and electron energies can change depending on conditions. However, the metallic conductor itself does not undergo a compositional change. Hence, the composition of the metallic conductor remains unchanged during metallic conductance.

According to NCERT, electrolytic conductance occurs through the movement of ions in solution. When direct current passes through an ionic solution for a prolonged period, ions migrate toward oppositely charged electrodes. At these electrodes, oxidation and reduction reactions take place. These electrochemical reactions consume certain ions and may produce new chemical species. As a result, the composition of the solution gradually changes. This process is known as electrolysis and is responsible for the chemical effects of electric current in electrolytic solutions. The passage specifically mentions that changes in composition occur because of electrochemical reactions. Therefore, electrochemical reactions at the electrodes are responsible for the observed change in composition.

Conductivity measures the ability of a material to conduct electric current. The greater the conductivity value, the better the conductor. Given values: 1. Graphite = 1.2 × 10⁵ S m⁻¹ 2. Silver = 6.2 × 10⁷ S m⁻¹ 3. Sodium = 2.1 × 10⁷ S m⁻¹ 4. Copper = 5.9 × 10⁷ S m⁻¹ Comparing the values: 6.2 × 10⁷ > 5.9 × 10⁷ > 2.1 × 10⁷ > 1.2 × 10⁵ Therefore, the decreasing order is: Silver > Copper > Sodium > Graphite Corresponding to: 2 > 4 > 3 > 1 Silver is one of the best metallic conductors, followed closely by copper. Sodium also conducts well but less effectively than silver and copper. Graphite conducts electricity because of its delocalized electrons, but its conductivity is much lower than that of metals.

Insulators are materials that strongly resist the flow of electric current. According to NCERT, materials such as glass, rubber and Teflon possess extremely low conductivity because they contain very few mobile charge carriers. A conductivity value of 1.0 × 10⁻¹⁶ S m⁻¹ is characteristic of a very good insulator. Glass is commonly cited as an insulating material with conductivity in this extremely low range. Because electrons are tightly bound within the structure, electric current cannot pass through it easily. This property makes glass useful in electrical insulation applications and laboratory equipment. Therefore, among the given options, glass is the material identified with conductivity approximately equal to 1.0 × 10⁻¹⁶ S m⁻¹.

Semiconductors such as silicon and germanium possess electrical conductivity intermediate between conductors and insulators. According to NCERT, the conductivity of semiconductors can be significantly increased by introducing small amounts of suitable impurities. This controlled addition of impurities is called doping. Doping creates additional charge carriers within the semiconductor crystal. Depending on the impurity added, either electrons or holes become the major charge carriers. This greatly enhances the conductivity of pure silicon. Doped semiconductors are the foundation of modern electronic devices such as transistors, integrated circuits and solar cells. The process is carefully controlled so that the electrical properties of the material can be adjusted according to the desired application. Therefore, the introduction of specific impurities to increase the conductivity of silicon is known as doping.

Semiconductors are materials whose conductivity lies between that of conductors and insulators. Common examples include silicon, germanium and gallium arsenide. These materials are extensively used in modern electronics because their conductivity can be controlled. Statement 1 is correct because silicon and germanium are standard semiconductors. Statement 2 is correct because doping introduces additional charge carriers and changes conductivity. Statement 4 is correct because gallium arsenide is an important semiconductor used in high-speed electronic and optoelectronic devices. Statement 3 is incorrect because semiconductors do not possess zero resistivity. Zero resistivity is associated with superconductors under special conditions. Semiconductors retain finite resistance and their conductivity depends on temperature and impurity concentration. Therefore, Statements 1, 2 and 4 are correct.

According to NCERT, the conductivity of an electrolytic solution depends strongly on the number of ions present in the solution. Strong electrolytes such as sodium chloride dissociate almost completely into Na⁺ and Cl⁻ ions when dissolved in water. As a result, a large number of charge carriers become available for the conduction of electricity. Weak electrolytes such as acetic acid ionize only partially. Therefore, only a small fraction of the dissolved molecules produce ions. Because the concentration of charge carriers is lower, conductivity is also lower. Since NaCl provides a much greater number of mobile ions than acetic acid at the same concentration, its conductivity is significantly higher. Thus, complete dissociation is the primary reason for the higher conductivity of strong electrolytes.

According to NCERT, the conductivity of electrolytic solutions depends upon multiple factors. These factors are presented in a particular order while discussing ionic conductance. The sequence begins with the nature of the electrolyte added because different electrolytes produce different numbers and types of ions. The next factor is the size of the ions produced and their degree of solvation, which influence ionic mobility. The third factor is the nature of the solvent and its viscosity, since viscosity affects the ease with which ions move through the solution. Finally, the concentration of the electrolyte influences the total number of ions available for conduction. Thus, the order presented in the text is: Nature of electrolyte → Size and solvation of ions → Nature and viscosity of solvent → Concentration of electrolyte.

Electrolysis is the process in which electrical energy is used to drive a non-spontaneous chemical reaction. In an electrolytic solution, ions move toward oppositely charged electrodes when an electric current is passed through the solution. At the electrodes, oxidation and reduction reactions occur. According to NCERT, electrolysis is a fundamental electrochemical process and is responsible for many industrial applications such as electroplating, extraction of metals and purification of metals. The movement of ions and the accompanying electrode reactions result in chemical changes that would not occur spontaneously under ordinary conditions. Since electrical energy is converted into chemical energy during this process, electrolysis represents a classic example of an electrochemical reaction. Therefore, the correct answer is electrolysis.

According to NCERT, conductivity of electrolytic solutions depends on several factors including the size of ions produced and their solvation. Solvation refers to the association of solvent molecules with ions present in solution. When ions become highly solvated, their effective size increases and their mobility generally decreases. Statement 1 is correct because solvation directly affects ionic mobility and conductivity. Statement 2 is incorrect because metallic conductors conduct through electrons and their composition remains unchanged during conduction. Statement 3 is incorrect because AC current is supplied by an external electrical source such as an oscillator and not by solvation. Statement 4 is incorrect because solvation strongly depends on the nature of the solvent and the interaction between solvent molecules and ions. Therefore, only Statement 1 is correct.

NCERT explains that direct current cannot be used conveniently for measuring the resistance of ionic solutions because prolonged passage of DC causes electrochemical reactions. These reactions alter the composition of the solution. To avoid this problem, alternating current is used. AC continuously reverses direction and minimizes net chemical change at the electrodes. Therefore, AC resolves the issue associated with DC measurements. Solid conductors such as metallic wires can be connected directly into a Wheatstone bridge circuit because their composition remains unchanged during current flow. Ionic solutions, however, cannot simply replace a metallic wire and must be placed inside a conductivity cell designed for resistance measurements. Thus, the correct matching is: 1-c, 2-d, 3-b, 4-a.

When resistance of an ionic solution is measured, direct current is unsuitable because it causes electrolysis and electrode polarization. Therefore, alternating current is used. According to NCERT, an oscillator supplies AC power in the audio-frequency range. The frequency typically used lies between 550 and 5000 cycles per second. At these frequencies, ions continuously reverse their direction of motion, reducing the possibility of chemical changes at the electrodes. This allows accurate measurement of resistance using a Wheatstone bridge arrangement. The oscillator is therefore an essential component of conductivity measurement apparatus. The specified frequency range ensures reliable measurements while minimizing errors arising from polarization effects.

NCERT describes the use of platinized platinum electrodes in conductivity cells. Platinization refers to the deposition of a thin layer of finely divided metallic platinum, commonly called platinum black, onto the surface of platinum electrodes through an electrochemical process. The platinum black coating increases the effective surface area of the electrodes and reduces polarization effects during conductivity measurements. This improves the accuracy and reliability of resistance measurements in electrolytic solutions. Platinized electrodes are therefore preferred over smooth platinum surfaces because they provide better contact with the electrolyte and minimize measurement errors. Thus, platinization specifically refers to electrochemical deposition of finely divided metallic platinum.

The Wheatstone bridge is an electrical circuit used to determine an unknown resistance accurately. In a balanced Wheatstone bridge, no current flows through the detector branch. Under this condition, the ratio of resistances in one arm equals the ratio of resistances in the other arm. The balance condition is: Rearranging: This equation allows determination of the unknown resistance of a conductivity cell. Once the resistance is known, conductivity and related parameters can be calculated. The Wheatstone bridge remains one of the most accurate methods for resistance measurement in electrochemical experiments.