Pressure Units: Videos & Practice Problems

Pressure, measured in pascals (Pa), is a fundamental concept in chemistry, particularly when studying gases. Gases consist of molecules moving randomly and exerting force on the walls of their container. This force per unit area is defined as pressure, with the equation:

$\mathrm{pressure}=\frac{\mathrm{force}}{\mathrm{area}}$

While the SI unit for pressure is the pascal, chemists often use non-SI units such as atmospheres (atm), millimeters of mercury (mmHg), and torr, with the relationships:

$1\mathrm{atm}=760\mathrm{mmHg}=760\mathrm{torr}$

Other units like kilopascals (kPa), bars, and psi are also used, with their respective values being:

$1.01325\times {10}^5\mathrm{Pa}for1\mathrm{atm},1.01325\mathrm{bars},14.696\mathrm{psi}$

Understanding these units and conversions is crucial for comprehending the behavior of gases in both ideal and non-ideal states.

Pressure Units

The SI unit for pressure is the Pascal (Pa), named after the French mathematician Blaise Pascal. Pressure can be understood as the force exerted by gas molecules on the walls of their container. In a gas, molecules move randomly in straight lines, colliding with each other and the container's surfaces. This random motion leads to the concept of pressure.

To quantify pressure, we use the formula:

\( P = \frac{F}{A} \)

where \( P \) represents pressure in Pascals, \( F \) is the force in newtons (N), and \( A \) is the area in square meters (m²). This relationship highlights that pressure is the force applied by gas molecules distributed over a specific area.

Understanding pressure is crucial as it lays the groundwork for exploring more complex concepts such as ideal gases, which follow specific laws under certain conditions, and non-ideal gases, which exhibit behaviors that deviate from these laws due to interactions between molecules.

Change in Pressure Units Example

When gas molecules are transferred from a smaller container to a larger one, the behavior of pressure can be understood through the relationship defined by the equation for pressure:\[P = \frac{F}{A}\]where \( P \) represents pressure, \( F \) is the force exerted by the gas molecules, and \( A \) is the area of the container's walls. In this scenario, the number of gas molecules remains constant, which means the force they exert does not change. However, when the gas is moved from a 5-liter container to a 10-liter container, the area \( A \) increases because the volume of the container has doubled.As the area increases while the force remains constant, the pressure must decrease. This is because pressure is inversely proportional to the area when the force is constant. Therefore, the correct conclusion is that the pressure will decrease when the same amount of gas is transferred to a larger volume. Thus, the best option in this case is that the pressure decreases.

Non-SI Pressure Units

In chemistry, pressure is a crucial concept, and several non-SI units are commonly used to express it. The most significant units include atmospheres (atm), millimeters of mercury (mmHg), and torr. Understanding the relationships between these units is essential, especially since they often appear in exams without being explicitly provided.

To memorize, it's important to note that:

• 1 atm = 760 mmHg
• 1 atm = 760 torr

These values are typically highlighted in educational materials, indicating their importance for memorization. In addition to these, other pressure units such as pascals (Pa), kilopascals (kPa), bars, and pounds per square inch (psi) are also used, though they are less frequently required to be memorized. Instead, they are often provided in question prompts or formula sheets.

The pressure values for these additional units are as follows:

• 1 atm = 101.325 kPa = 101325 Pa
• 1 bar = 100 kPa = 100000 Pa
• 1 psi = 14.696 psi

These relationships allow for conversions between different pressure units, facilitating calculations in various chemical contexts. While the first three units (atm, mmHg, and torr) are the most commonly used in chemistry, familiarity with the others can enhance understanding and application in practical scenarios.

Pressure Conversion Example

To convert the average pressure in Denver, Colorado, from inches of mercury to millimeters of mercury and then to atmospheres, we can utilize dimensional analysis. The average pressure is given as 24.9 inches of mercury. The first step is to convert inches of mercury to centimeters of mercury, using the conversion factor that 1 inch equals 2.54 centimeters.

Starting with the pressure:

24.9 \text{ inches of mercury} \times \frac{2.54 \text{ cm}}{1 \text{ inch}} = 63.246 \text{ cm of mercury}

Next, we convert centimeters of mercury to millimeters of mercury, knowing that 1 centimeter equals 10 millimeters:

63.246 \text{ cm of mercury} \times \frac{10 \text{ mm}}{1 \text{ cm}} = 632.46 \text{ mm of mercury}

Since the original value has three significant figures, we round this to:

632 \text{ mm of mercury}

Now, to convert millimeters of mercury to atmospheres, we use the conversion factor that 1 atmosphere is equal to 760 millimeters of mercury:

632 \text{ mm of mercury} \times \frac{1 \text{ atm}}{760 \text{ mm}} = 0.832 \text{ atm}

Thus, the final results are:

Pressure in millimeters of mercury: 632 mm of mercury

Pressure in atmospheres: 0.832 atm

This process illustrates how dimensional analysis can effectively convert between different units of pressure.

Here’s what students ask on this topic:

What is pressure?

Pressure is a physical quantity that expresses the force exerted per unit area on the surface of an object. It is a scalar quantity, meaning it has magnitude but no direction. In the International System of Units (SI), pressure is measured in pascals (Pa), where one pascal is equivalent to one newton per square meter. Pressure can be caused by gases, liquids, or solids.

In the context of gases and liquids, we often refer to it as the force that these substances exert on the walls of their container, and it can be due to their weight or their motion. In solids, pressure can be due to external forces acting on the surface of an object. For example, when you press your hand against a wall, you are applying pressure to the wall's surface.

Pressure is a fundamental concept in various fields, including physics, engineering, and meteorology. It plays a critical role in understanding how forces are distributed over areas, which is essential for the analysis of fluid flow, the design of structural elements, and predicting weather patterns.

What are the units of pressure?

Pressure is defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Various units are used to measure pressure, and here are some of the most common ones:

1. Pascal (Pa) - The SI (International System of Units) unit of pressure, defined as one newton per square meter (N/m²).
2. Bar - Commonly used in meteorology and for industrial pressure measurements, where 1 bar is equivalent to 100,000 pascals or 0.1 megapascals (MPa).
3. Atmosphere (atm) - A unit of pressure based on the average atmospheric pressure at sea level, where 1 atm is equivalent to 101,325 pascals.
4. Torr - Named after Evangelista Torricelli, where 1 torr is approximately equal to the pressure exerted by a millimeter of mercury (mmHg), or 133.322 pascals.
5. Pounds per square inch (psi) - A unit of pressure common in the United States, measuring the force in pounds applied to one square inch of area.

Each of these units can be converted into the others using conversion factors, allowing for flexibility in how pressure is represented depending on the context or industry standards.

What is the SI unit of pressure?

The SI (International System of Units) unit of pressure is the pascal, which is abbreviated as Pa. One pascal is defined as one newton per square meter (N/m²). This unit is named after Blaise Pascal, a French mathematician, physicist, and inventor. In practical terms, pressure indicates the amount of force applied over a unit area. In various scientific and engineering contexts, other units such as bar, atmosphere (atm), and torr (mmHg) are also used, but the pascal remains the SI standard. When measuring atmospheric pressure, for example, you might encounter the hectopascal (hPa), which is equivalent to 100 pascals and commonly used in meteorology.