Question

What is the unit of Henry's law constant?

Answer 1

Henry's Law Constant (kH):

Henry's law states that the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of the gas above the liquid. The constant of proportionality is known as Henry's law constant, denoted as \( k_H \). The units of \( k_H \) depend on the units used for the partial pressure of the gas and the concentration of the gas in the liquid phase.

1. When the partial pressure of the gas is in atmospheres (atm) and the concentration is in moles per liter (M):

In this case, the units for partial pressure are in atmospheres (atm), and the units for concentration are in moles per liter (M). Henry's law can be expressed as:

\[ \text{Concentration of gas (M)} = k_H \times \text{Partial pressure (atm)} \] Rearranging to find the units of \( k_H \), we get: \[ k_H = \frac{\text{Concentration of gas (M)}}{\text{Partial pressure (atm)}} \] 

2. When the partial pressure of the gas is in pascals (Pa) and the concentration is in moles per cubic meter (mol/m³):

In this case, the units for partial pressure are in pascals (Pa), and the units for concentration are in moles per cubic meter (mol/m³). Henry's law can be written as:

 

\[ \text{Concentration of gas (mol/m³)} = k_H \times \text{Partial pressure (Pa)} \] Rearranging for \( k_H \), we get: \[ k_H = \frac{\text{Concentration of gas (mol/m³)}}{\text{Partial pressure (Pa)}} \] Since the concentration is in moles per cubic meter (mol/m³), the units for \( k_H \) are: \[ \text{Units of } k_H = \frac{\text{mol}}{\text{m}^3 \cdot \text{Pa}} \] Therefore, the units of Henry's law constant \( k_H \) are \( \text{m}^3 \cdot \text{Pa/mol} \).

Conclusion:

• If the partial pressure is in atmospheres (atm) and concentration in moles per liter (M), then the units of \( k_H \) are \( \text{L·atm/mol} \).
• If the partial pressure is in pascals (Pa) and concentration in moles per cubic meter (mol/m³), then the units of \( k_H \) are \( \text{m}^3 \cdot \text{Pa/mol} \).