Answer:
2.01
Step-by-step explanation:
The effusion is the passage of the molecules by a small hole by a difference of pressure. By Graham's Law, the rate of the effusion is inversely proportional to the square of the molar mass of the compound. Thus,
rateHF/rateHBr = √MHBr /√MHF
MHBr = 81 g/mol
MHF = 20 g/mol
rateHF/rateHBr = √81/√20
rateHF/rateHBr = 2.01