How to calculate mass using PV=nRT equation

Naphthalene, C10H8, is the substance present in some moth balls. Its vapor pressure at 25° is 0.300mmHg
a. how many milligrams f naphthalene will sublime into an evacuated 1.000-L flask?

At 25oC, solid naphthalene will be in equilibrium with its vapor, which will be at a pressure of 0.300 mmHg.
You have
PV = nRT
T = 298 K
P = 0.300 mmHg
V= 1.00 L
If you find R with matching units (or convert units as needed) you can solve for the number of moles, n.
You can convert number of moles to mass, by using the fact that 1 mole of C10H8should have a mass of 128.2 grams.
(10 x 12.01 + 8 x 1.01 = 128.2)