HENRY'S LAW FORMULA

HENRY'S LAW FORMULA

About Henry's Law Formula We are familiar that gases are completely miscible with each other. Gases also dissolve in liquids and solids. For example, soda water contains carbon dioxide dissolved in water under high pressure. Oxygen is sufficiently soluble in water to allow the survival of aquatic life in lakes, rivers, and oceans. An example of the dissolution of gas in a solid is the solubility of hydrogen gas in palladium.

The solubility of a gas in a liquid is determined by several factors. In addition to the nature of the gas and the liquid, the solubility of the gas depends on the temperature and pressure of the system. The solubility of a gas in a liquid is governed by Henry’s law which states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas. Dalton, a contemporary of Henry, also concluded independently that the solubility of a gas in a liquid solution is a function of the partial pressure of the gas. If we use the mole fraction of the gas in the solution as a measure of its solubility, then: The mole fraction of the gas in a solution is proportional to the partial pressure of the gas. For More Chemistry Formulas check out the Main page of Physics Wallah.

Or, partial pressure of the gas in solution = KH mole fraction of the gas in solution Here KH is Henry’s law constant

or, p = KHx If we draw a graph between the partial pressure of the gas versus the mole fraction of the gas in solution, then we should get a plot of the type as shown in the figure.

Different gases have different KH values at the same temperature. This suggests that KH is a function of the nature of the gas. The table gives KH values of some common gases at a specified temperature Henry’s law finds several applications in industry and explains some biological phenomena. Notable among these are: ● To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure. ● To minimize the painful effects accompanying the decompression of deep sea divers, oxygen diluted with less soluble helium gas is used as breathing gas. ● In the lungs, where oxygen is present in the air with high partial pressure, hemoglobin combines with oxygen to form oxyhemoglobin. In tissues where the partial pressure of oxygen is low, oxyhemoglobin releases oxygen for utilization in cellular activities.

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Values of Henry’s law constant (KH) for some selected gases in water Gas. Temp/K He

293

KH/kbar 144.97

H2

293

69.16

N2 .

293

76.48

N2

303

88.84

O2 .

293

34.86

O2 .

393

46.82

It is obvious from the figure that the higher the value of KH at a given pressure, the lower the solubility of the gas in the liquid. It can be seen from the table that the KH value for both N2 and O2 increases with an increase in temperature indicating that the solubility of gases decreases with the increase in temperature. It is due to this reason that aquatic species are more comfortable in cold waters rather than warm waters.

Example of Henry's law If N2 gas is bubbled through water at 293 K, how many millimoles of N2 gas would dissolve in 1 liter of water? Assume that N¬2 exerts a partial pressure of 0.987 bar. Given that Henry’s law constant for N2 at 293 K is 76.84 kbar.

Solution: The solubility of gas is related to its mole fraction in the aqueous solution. The mole fraction of the gas in the solution is calculated by applying Henry’s law. Thus,

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