Answer
5.625 g/L
Work Step by Step
We have the equation d = PM/RT but first we need to convert everything to the proper units. The pressure is 707 torr so we convert it to atm:
707 torr * (1atm/760 torr) = 0.930 atm
Then the temperature is 21C so we add 273 to get Kelvin, which is 294K
Last we find the molar mass of Sulfur Hexaflouride, which is SF6. The mass of Sulfur is 32g/mol and F is 19g/mol, therefore, we have:
32 + 6*19 = 146 g/mol
Therefore, we have d = PM/RT = (0.930 atm * 146 g/mol) / (0.0821 * 294K) = 5.625 g/L