Answer
80.15 g/mol
Work Step by Step
We have the equation M = dRT/P were d is the density and M is the molar mass. We have that 2.50g occupies 0.875L so we divide the two to find the density in g/L is 2.857 g/L
Then we convert the 35C to Kelvin by adding 273, 308K.
Then we convert the 685 torr to atm:
685 torr * (1atm/760 torr) = 0.901 atm
Now we have M = (2.857 g/L * 0.0821 * 308K) / 0.901atm = 80.15 g/mol
