State Avogadro’s law. Use simple calculation to show that moist air is lighter than dry air. [1+2]
WBBSE
Class X
Physical Science
Behaviour of Gases
03 Mark
2025
Avogadro’s Law : Equal volumes of all gases at the same temperature and pressure contain an equal number of molecules.
To show that moist air is lighter than dry air
According to Avogadro’s Law, equal volumes of gases contain an equal number of molecules.
Molecular mass of dry air ≈ 29
Molecular mass of H₂O = (2 × 1) + 16 = 18
When water vapour mixes with dry air, some of the heavier air molecules (average molecular mass = 29) are replaced by lighter water vapour molecules (molecular mass = 18).
Since 18 < 29, the average molecular mass of moist air decreases.
Therefore, moist air is lighter than dry air.
Related Questions
More WBBSE Moderate level questions
1/12
Q.
Mention a phenomenon by which it can be said that the gas molecules are in...
A.
Phenomenon: Brownian motion shows that gas molecules are in constant motion. Two...
Q.
What is an ideal gas? What is the effect of the increase in temperature on...
A.
A gas which obeys Boyle’s law, Charles’ law and Avogadro’s law for...
Q.
Establish the combined form of Boyle’s law and Charles’ law.
A.
Let V be the volume of a given mass of gas at...
Q.
What is the number of molecules present in 16 g of O₂ at S.T.P.?
A.
3.01 × 10²³ molecules Explanation Molar mass of O₂ = 16 ×...
Q.
The volume of a certain amount of gas at a temperature of 27°C and at...
A.
Initial Pressure (P₁) = 76 cm Hg Initial Volume (V₁) = 200...
Q.
A certain amount of gas occupies 750 cc at –3°C. The gas is heated at...
A.
Initial Volume (V1) = 750 cc Initial Temperature (T1) = –3°C =...
Q.
The coefficient of linear expansion of aluminium is 24×10-6 ºC-1. What would be its value...
A.
Coefficient of linear expansion of aluminium = 24 × 10⁻⁶ °C⁻¹. Since...
Q.
Write down the correlation between absolute temperature and density of gas.
A.
Density (ρ) ∝ 1/Temperature (T), when pressure is constant.
Q.
The value of the ratio of the volume coefficient to the pressure coefficient of an...
A.
(d) 1 Explanation For an ideal gas, the ratio of the volume...
Q.
Draw I-V graph according to ohm’s law in the case of a metallic conductor.
A.
I-V graph
Q.
At a certain pressure the volume of a fixed mass of a gas at a...
A.
The increase in volume is Yo/273.
Q.
What is the value of PV for 4 g of H2 gas at STP? (H...
A.
b. 2 RT Explanation: Number of mole (n) = 4/2 = 2...