water molecule. We know that mole of water has a mass approximately equal to ( + )g = g = . kg. Since mole contains about × molecules (Avogadro’s number), the mass of a molecule of water is ( .
)/( × ) kg = × – kg. Therefore, a rough estimate of the volume of a water molecule is as follows : Volume of a water molecule = ( × – kg)/ ( kg m – ) = × – m = ( / ) π (Radius) Hence, Radius ≈ × - m = Å Example . What is the average distance between atoms (interatomic distance) in water? Use the data given in Examples .
and . . Answer : A given mass of water in vapour state has . × times the volume of the same mass of water in liquid state (Ex.
. ). This is also the increase in the amount of volume available for each molecule of water. When volume increases by times the radius increases by V / or times, i.e., × Å = Å.
So the average distance is × = Å. Example . A vessel contains two non- reactive gases : neon (monatomic) and oxygen (diatomic). The ratio of their partial pressures is : .
Estimate the ratio of (i) number of molecules and (ii) mass density of neon and oxygen in the vessel. Atomic mass of Ne = . u, molecular mass of O = . u.
Answer Partial pressure of a gas in a mixture is the pressure it would have for the same volume and temperature if it alone occupied the vessel. (The total pressure of a mixture of non-reactive gases is the sum of partial pressures due to its constituent gases.) Each gas (assumed ideal) obeys the gas law. Since V and T are common to the two gases, we have P V = µ RT and P V = µ