Final answer:
According to Graham's law, the rate of effusion of a gas is inversely proportional to the square root of its molar mass. Using this relationship, we can determine that the molar mass of substance A is 2.01 g/mol.
Step-by-step explanation:
Graham's law states that the rate of effusion of a gas is inversely proportional to the square root of its molar mass. If the rate of effusion of substance A is eight times that of substance B, we can set up the equation:
rate of effusion of A / rate of effusion of B = sqrt(molar mass of B / molar mass of A)
Given that the molar mass of substance B is 129 g/mol, we can substitute the values and solve for the molar mass of substance A:
8 = sqrt(129 / molar mass of A)
Squaring both sides of the equation, we get:
64 = 129 / molar mass of A
Multiplying both sides by the molar mass of A, we find:
molar mass of A = 129 / 64 = 2.01 g/mol