Solubility and Henry's Law MCQ - Practice Questions & Answers

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Solubility and Henry's Law

Solubility of a substance is its maximum amount that can be dissolved in a specified amount of solvent at a specified temperature. It depends upon the nature of solute and solvent as well as temperature and pressure. Let us consider the effect of these factors in solution of a solid or a gas in a liquid.

Solubility of a Solid in a Liquid
Every solid does not dissolve in a given liquid. While sodium chloride and sugar dissolve readily in water, naphthalene and anthracene do not. On the other hand, naphthalene and anthracene dissolve readily in benzene but sodium chloride and sugar do not. It is observed that polar solutes dissolve in polar solvents and non polar solutes in non-polar solvents. In general, a solute dissolves in a solvent if the intermolecular interactions are similar in the two or we may say like dissolves like.
When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution. This process is known as dissolution. Some solute particles in solution collide with the solid solute particles and get separated out of solution. This process is known as crystallisation. A stage is reached when the two processes occur at the same rate. Under such conditions, number of solute particles going into solution will be equal to the solute particles separating out and a state of dynamic equilibrium is reached.
$\text { Solute }+\text { Solvent } \rightleftharpoons \text { Solution }$
At this stage the concentration of solute in solution will remain constant under the given conditions, i.e., temperature and pressure. Similar process is followed when gases are dissolved in liquid solvents. Such a solution in which no more solute can be dissolved at the same temperature and pressure is called a saturated solution. An unsaturated solution is one in which more solute can be dissolved at the same temperature. The solution which is in dynamic equilibrium with undissolved solute is the saturated solution and contains the maximum amount of solute dissolved in a given amount of solvent. Thus, the concentration of solute in such a solution is its solubility.
Earlier we have observed that solubility of one substance into another depends on the nature of the substances. In addition to these variables, two other parameters, i.e., temperature and pressure also control this phenomenon.

Effect of temperature: The solubility of a solid in a liquid is significantly affected by temperature changes. Consider the equilibrium represented by the equation. This, being dynamic equilibrium, must follow Le Chateliers Principle. In general, if in a nearly saturated solution, the dissolution process is endothermic (∆_sol H > 0), the solubility should increase with rise in temperature and if it is exothermic (∆_sol H < 0) the solubility should decrease. These trends are also observed experimentally.
Effect of pressure: Pressure does not have any significant effect on solubility of solids in liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by changes in pressure.

Solubility of a Gas in a Liquid
Many gases dissolve in water. Oxygen dissolves only to a small extent in water. It is this dissolved oxygen which sustains all aquatic life. On the other hand, hydrogen chloride gas (HCl) is highly soluble in water. Solubility of gases in liquids is greatly affected by pressure and temperature. The solubility of gases increase with increase of pressure. For solution of gases in a solvent, consider a system as shown in figure given below. The lower part is solution and the upper part is gaseous system at pressure p and temperature T. Assume this system to be in a state of dynamic equilibrium, i.e., under these conditions rate of gaseous particles entering and leaving the solution phase is the same. Now increase the pressure over the solution phase by compressing the gas to a smaller volume. This will increase the number of gaseous particles per unit volume over the solution and also the rate at which the gaseous particles are striking the surface of solution to enter it. The solubility of the gas will increase until a new equilibrium is reached resulting in an increase in the pressure of a gas above the solution and thus its solubility increases.

Henry was the first to give a quantitative relation between pressure and solubility of a gas in a solvent which is known as Henry’s law. The law states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution. Dalton, a contemporary of Henry, also concluded independently that the solubility of a gas in a liquid solution is a function of partial pressure of the gas. If we use the mole fraction of a gas in the solution as a measure of its solubility, then it can be said that the mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution. The most commonly used form of Henry’s law states that “the partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution” and is expressed as:
$\mathrm{p=K_{\mathrm{H}} x}$
Here K_H is the Henry’s law constant. If we draw a graph between partial pressure of the gas versus mole fraction of the gas in solution, then we should get a plot of the type as shown in figure given below.

Different gases have different K_H values at the same temperature as shown in the given table. This suggests that K_H is a function of the nature of the gas. It is obvious from the above equation that higher the value of K_H at a given pressure, the lower is the solubility of the gas in the liquid. It can be seen from the given table that K_H values for both N₂ and O₂ increase with increase of temperature indicating that the solubility of gases increases with decrease of temperature. It is due to this reason that aquatic species are more comfortable in cold waters rather than in warm waters.

Gas	Temperature(K)	K_H/kbar
Helium	293	144.97
Hydrogen	293	69.16
Nitrogen	293	76.48
Nitrogen	303	88.84
Oxygen	293	34.86
Oxygen	303	46.82
Argon	298	40.3
Carbon dioxide	298	1.67
Formaldehyde	298	1.83x10^-5
Methane	298	0.413
Vinyl chloride	298	0.611

Real Life Examples of Henry's Law

There are various real-life examples of Henry's law. Some of them are mentioned below:

Soda bottle fizzes when opened: When the soda bottle is open, then the pressure decreases in the bottle. Now, due to this decrease in pressure, the solubility of gas decreases. Now if we leave this bottle open for some time, then all fizz goes out and we do not feel the drinking.
Anoxia: This is the condition of tiredness and mental confusion at higher altitudes. At higher altitudes, pressure decreases and thus the solubility of oxygen gas in the body decreases which causes anoxia.
Scuba divers: When scuba divers go deep into the ocean, then pressure increases. Now due to this increase in pressure the solubility of gases in the blood increases. Further, when these divers come up at the sea level, then pressure decreases and solubility of gases in the blood decreases. Due to this decrease in solubility, the gases come out of the capillaries in the form of bubbles which causes a serious medical condition.