In this question, we have to use Graham's law of effusion in order to find the molar mass of this unknown gas, the formula is the following:
r1/r2 = √ M2/M1
Where:
r1 = rate of gas 1, in this case oxygen, 625 m/sec
r2 = rate of gas 2, unknown gas, 425 m/sec
M1 = molar mass of gas 1, oxygen, 32g/mol
M2 = molar mass of gas 2, unknown
Adding the values into the formula:
625/425 = √ M2/32
1.47 = √ M2/32
(1.47)^2 = M2/32
2.16 = M2/32
M2 = 32 * 2.16
M2 = 69.12g/mol is the molar mass of this gas