How many millimeters of cl2 gas must you have to obtain 0.20 g at stp?

STP = Standard Temperature (273 K) Pressure (1 atm)

we also have the mass of cl2, 0.2g. We use this to find the number of moles.

Use the ideal gas law.
PV = nRT
P= pressure in atm
V= volume in Liters
n = number of moles of gas
R = 0.082058
T = Temperature in Kelvin

P = 1 atm
V = ? (we are looking for this)
n = moles
   = mass/molar mass
   = 0.2g/(2*35.453)
   = 0.0028 moles
R = 0.082058
T = 273 K

(1)V = (0.0028)(0.082058)(273)
V = 0.063 L
= 60 mL (1 sig fig)

good luck!