A certain gas has a density of 1.30 g/L at STP. What will its density, in g/L, be at 546 K and 380 torr pressure ?
The gas will have the same density at 546 K and 380 torr that it had at STP.
There are two ways to prove this - a longer one and a shorter one. Here's the longer one.
Standard Temperature and Pressure - STP - is defined as having a temperature of
The number of moles is defined as the ratio between the gas' mass and its molar mass; plug this equation into the above one
Density is defined as mass per unit of volume,
If you write the equation for the non-STP state, you'll get the exact same thing ,except you'll now have
This means that
In you case,
Now for the shorter answer. Notice that the pressure drops by half and the temperature doubles; if you take into account the fact that the number of moles is constant, the obvious conclusion is that volume remains constant as well.
SInce density is mass per volume, and neither mass nor volume change, then the density will be the same for both the STP and the non-STP conditions.