Ans. is 'b' i.e. V/T = constant Physical behavior of gases Gas pressure o The gas molecules are in continuous motion, moving through space at high velocity' and being deflected from their course only upon collision with other gas molecules or with boundries of the space in which they are contained. It would be obvious that greater the concentration of the molecules of the gas, the greater wrould be the force exerted by the molecules against the container at any given time. Therefore, the pressure of a gas is directly proportionate to its concentration. Boyle's law o At a constant temperature, the pressure (P) of a given mass is inversely proportionate to its volume (p a 1/v), i.e., PV = constant. Dalton's law of partial pressure o When a mixture of gases is present in a container, each gas exerts a pressure according to its own concentration, independent of the concentration of other gases present in the mixture. In other words, each gas behaves as if it was present alone. The pressure of each gas in mixture of gases is known as its partial pressure or tension. For example, environment air is a mixture of 21 % O2 and 79% N2. Therefore, the partial pressure of O2 (PO2) in the environmental air, at sea level (barometric pressure 760 mm Hg), would be 21 /100 x 760 = 160 mm Hg. Similarly, partial pressure of N2 (pH2) in the environmental air, at sea level, wrould be 79/100 x 760 = 600 mm Hg. Total pressure = pO2 + pN2 = 760 mm Hg. Water vapour pressure o The inhaled air is humified by the water vapours from the conducting passages. By the time it reaches the alveoli, it is saturated with water vapour. Therefore, in the alveolar air, besides N2 and O2, water vapour also exerts its partial pressure, which is 47 mm Hg. It is not affected by the presence or absence of other gases, or the atmospheric pressure. That means, there is a constant water pressure of 47 mm Hg once the air reaches the alveoli (in alveolar pressure). Partial pressure of gas dissolved in water and tissues o When a gas is brought in contact writh water, (or body tissues), some of the gas molecules move into water and get dissolved in it. But the dissolved gas molecules also tend to escape from the water. After a time the rate of escape equals rate of entry, i.e., equilbrium is reached. Henery's law states that the concentration of a gas in solution at equilbrium is directly proportional to its partial pressure in gas phase : - Concentration of dissolved gas = Partial pressure x Solubility coefficient o Solubility coefficient (absorption coefficient) of CO2 is 20 times greater than O2. Charles's law When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be directly related. this directly proportional relationship can be written as: or V/T = k Avogadro's hypothesis o Different gases at the same pressure and temperature contain the same number of molecules. Thus a gram molecule of any gas occupies a volume of 22.4 liters at STPD (00C, 760 m Hg pressure, dry). Gas equation o It is PV= nRT(P = pressure. V = volume, n = gram molecules of gas. R = gas constant and T = absolute temperature)