Henry's Law Calculations: Videos & Practice Problems

The concentration of a dissolved gas, often referred to as its solubility, can be calculated using Henry's Law, which relates the solubility of a gas to its partial pressure. The key variable in this relationship is Henry's law constant, denoted as \( k_h \). This constant indicates the solubility of a gas at a specific temperature in a given solvent, expressed in molarity (M). It's important to note that the terms concentration and molarity are often used interchangeably in academic settings.

According to Henry's Law, the solubility of a gas (\( s_{\text{gas}} \)) in molarity can be determined using the formula:

\( s_{\text{gas}} = k_h \times P_{\text{gas}} \)

In this equation, \( P_{\text{gas}} \) represents the partial pressure of the gas, typically measured in atmospheres (atm). The units for Henry's law constant can vary; it is commonly expressed in molarity per atmosphere (M/atm), but may also appear in other pressure units such as torr or millimeters of mercury (mmHg). To simplify calculations, it is advisable to convert all pressure measurements to atmospheres when using the standard form of Henry's law constant.

Understanding the relationship between solubility, Henry's law constant, and partial pressure is crucial for applications in chemistry, particularly in fields involving gas solubility in liquids.

Henry's Law describes the relationship between the solubility of a gas in a liquid and the pressure of that gas above the liquid. Specifically, the 2.4 method of Henry's Law is particularly useful when analyzing how changes in pressure influence gas solubility. This formula is applicable when comparing two different solubilities at two different pressures for a specific gas.

The formula can be expressed as:

\(\frac{S_1}{P_1} = \frac{S_2}{P_2}\)

In this equation, \(S_1\) represents the initial solubility of the gas, \(P_1\) is the initial pressure, \(S_2\) is the final solubility, and \(P_2\) is the final pressure. The solubility can be measured in various units, including molarity (moles per liter) or mass per volume (such as grams per liter).

Understanding this relationship is crucial for applications in fields such as chemistry, environmental science, and engineering, where gas solubility plays a significant role in processes like gas exchange in aquatic systems and the design of chemical reactors.

To determine the new pressure of dichloromethane (CH₂Cl₂) when its solubility changes, we can use Henry's Law, which states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid. This relationship can be expressed mathematically as:

\[ \frac{s_1}{p_1} = \frac{s_2}{p_2} \]

In this equation, s represents solubility (in milligrams per liter), and p represents pressure (in atmospheres). Here, we have:

• s₁ = 0.384 mg/L at p₁ = 2.88 atm
• s₂ = 0.225 mg/L at p₂ (unknown)

To find p₂, we can rearrange the equation:

\[ p_2 = \frac{s_2 \cdot p_1}{s_1} \]

Substituting the known values into the equation gives:

\[ p_2 = \frac{0.225 \, \text{mg/L} \cdot 2.88 \, \text{atm}}{0.384 \, \text{mg/L}} \]

Calculating the right side:

\[ p_2 = \frac{0.648 \, \text{mg atm/L}}{0.384 \, \text{mg/L}} \approx 1.69 \, \text{atm} \]

Thus, the new pressure p₂ is approximately 1.69 atmospheres. This demonstrates how changes in solubility are directly related to changes in pressure, illustrating the principles of gas solubility in liquids.