Question

A constant-volume gas thermometer registers an absolute pressure corresponding to 325 mm of mercury when in contact with water at the triple point. What pressure does it read when in contact with water at the normal boiling point?

Short answer

At the boiling point, the pressure is about 444.6 mmHg.

Step-by-step solution

Understand the Triple Point of Water

The triple point of water is a specific thermodynamic state where water coexists in three phases: solid, liquid, and gas. It is a fixed point used to define the Kelvin temperature scale, and it occurs at a temperature of 273.16 K and a specific pressure.

Define the Pressure and Temperature Relationship

The pressure of an ideal gas is directly proportional to its temperature when volume is constant, described by the equation \( P = kT \), where \( P \) is the pressure, \( T \) is the absolute temperature in Kelvin, and \( k \) is a constant of proportionality.

Determine the Initial State for the Triple Point

At the triple point of water, the pressure is given as 325 mmHg, and the temperature is 273.16 K.

Set Up the Proportionality Equation

Using \( P_1/T_1 = P_2/T_2 \), where \( P_1 = 325 \, \text{mmHg} \) and \( T_1 = 273.16 \, \text{K} \), and \( T_2 = 373.15 \, \text{K} \) at the boiling point of water, we aim to find \( P_2 \).

Solve for Pressure at Boiling Point

Re-arrange the equation to find \( P_2 \):\[P_2 = \frac{P_1 \times T_2}{T_1}\]Substitute the values:\[P_2 = \frac{325 \, \text{mmHg} \times 373.15 \, \text{K}}{273.16 \, \text{K}}\]Calculate \( P_2 \):\[P_2 \approx 444.6 \, \text{mmHg}\]

Interpret the Final Result

At the normal boiling point of water, which is 373.15 K, the constant-volume gas thermometer reads approximately 444.6 mmHg.

Key concepts

Triple Point of Water

The triple point of water is a unique condition where water exists simultaneously as a solid, liquid, and gas. This equilibrium state is pivotal in thermodynamics because it serves as a reference point for defining the Kelvin temperature scale. Specifically, the triple point occurs at a temperature of 273.16 Kelvin and a specific pressure. This point is important for scientific measurements because it provides a precise standard, ensuring consistency across various experimental conditions. Understanding the triple point helps in grasping various thermodynamic processes and how substances behave under different pressures and temperatures.
It ensures that measurements taken with devices like the constant-volume gas thermometer are accurate and reliable.

Pressure-Temperature Relationship

The relationship between pressure and temperature in an ideal gas thermometer, such as a constant-volume gas thermometer, is governed by direct proportionality. This implies that at a constant volume, if the temperature of the gas increases, the pressure also increases, and vice versa. Mathematically, this relationship is captured by the expression \( P = kT \), where \( P \) signifies the pressure, \( T \) is the absolute temperature in Kelvin, and \( k \) is a constant of proportionality.
This formula is crucial for predicting how changes in temperature can affect pressure readings. It is also foundational in understanding the behavior of gases in closed systems, which is vital for many scientific and engineering applications. When using a constant-volume gas thermometer, this relationship allows for accurate temperature measurements based on observed pressure changes.

Ideal Gas Law

The ideal gas law is a fundamental equation in chemistry and physics that correlates the pressure, volume, temperature, and amount of a gas. Although not directly used in solving the exercise, understanding this law provides insight into why the principles applied to the constant-volume gas thermometer hold true. The law is usually written as \( PV = nRT \), where \( P \) represents pressure, \( V \) is volume, \( n \) is the amount of gas in moles, \( R \) is the ideal gas constant, and \( T \) is the temperature in Kelvin.
However, when we limit the scenario to constant volume and a fixed amount of gas, the relationship simplifies to the direct proportionality used in the exercise. Thus, the concepts laid out by the ideal gas law underscore the simplified equation \( P = kT \) used with a gas thermometer. It's important for providing a theoretical basis for observed behavior and confirming the accurate assessment of temperature using a known pressure.

Thermodynamic State

A thermodynamic state refers to the particular conditions—such as pressure, temperature, and volume—that define the status of a system. In the context of the constant-volume gas thermometer, understanding the thermodynamic state is crucial for interpreting and predicting pressure readings at various temperatures. Each point of consistent conditions, like the triple point of water or the boiling point, represents a unique thermodynamic state.
When comparing these states, one can use the relationship between pressure and temperature to predict changes or outcomes, as seen in the problem. Identifying and understanding these states help in analyzing and solving thermodynamic problems efficiently. By using constants like the triple point in calculations, scientists can reliably assess how a system will behave under expected conditions, broadening our understanding of material behavior in various environments.