P + P + … ( . ) Clearly P = µ R T/V is the pressure that gas would exert at the same conditions of volume and temperature if no other gases were present. This is called the partial pressure of the gas. Thus, the total pressure of a mixture of ideal gases is the sum of partial pressures.
This is Dalton’s law of partial pressures. Fig. . Experimental T-V curves (solid lines) for CO at three pressures compared with Charles’ law (dotted lines).
T is in units of K and V in units of . litres. We next consider some examples which give us information about the volume occupied by the molecules and the volume of a single molecule. Example .
The density of water is kg m – . The density of water vapour at ° C and atm pressure is . kg m – . The volume of a molecule multiplied by the total number gives ,what is called, molecular volume.
Estimate the ratio (or fraction) of the molecular volume to the total volume occupied by the water vapour under the above conditions of temperature and pressure. Answer For a given mass of water molecules, the density is less if volume is large. So the volume of the vapour is / . = /( × - ) times larger.
If densities of bulk water and water molecules are same, then the fraction of molecular volume to the total volume in liquid state is . As volume in vapour state has increased, the fractional volume is less by the same amount, i.e. × - . Example .
Estimate the volume of a water molecule using the data in Example . . Answer In the liquid (or solid) phase, the molecules of water are quite closely packed. The density of water molecule may therefore, be regarded as roughly equal to the density of bulk water = kg m – .
To estimate the volume of a water molecule, we need to know the mass of a single