define Grahams Law ?

Graham''s Law   
The rates of effusion of gases are inversely proportional to the square roots of  
their molecular weights or densities. 

 Graham''s Law is a relation which states that the rate of the effusion of a gas is inversely proportional to the square root of its density or molecular mass.

Rate1 / Rate2 = (M2 / M1)^1/2

where:
Rate1 is the rate of effusion of one gas, expressed as volume or as moles per unit time.
Rate2 is the rate of effusion of the second gas.
M1 is the molar mass of gas 1.
M2 is the molar mass of gas 2.

Graham''s law states that the rate of effusion of a gas is inversely proportional to its molecular weight.

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Graham''''s Law   
The rates of effusion of gases are inversely proportional to the square roots of  
their molecular weights or densities.