What is the molar mass of a compound if a gaseous sample has a density of 2.807 g/L at 18 °C and 728 torr?

= 728/(2.807)(0.08206)(291.15)

=10.855 g

?

P*molar mass = density*RT

Solve for molar mass.

Nope. P must be in atmospheres. 728/760 = ? atm.

Also it appears to me you have set up the reciprocal.
M = dRT/P

To determine the molar mass of a compound based on its density, you need to follow a few steps:

Step 1: Convert temperature to Kelvin.
To convert 18 °C to Kelvin, you add 273.15 to the Celsius temperature:
T (Kelvin) = 18 °C + 273.15 = 291.15 K

Step 2: Convert pressure to atm.
Given that the pressure is given in torr, you need to convert it to atm:
Pressure (atm) = 728 torr / 760 torr = 0.958 atm

Step 3: Use the density to calculate molar mass.
The ideal gas law equation, PV = nRT, can be rearranged to solve for molar mass (M):
Molar Mass (M) = (density × RT) / P

Where:
R = ideal gas constant (0.0821 L·atm/mol·K)
T = temperature in Kelvin
P = pressure in atm

Plugging in the values:
M = (2.807 g/L × 0.0821 L·atm/mol·K × 291.15 K) / 0.958 atm

Calculating this expression gives you the molar mass of the compound.