We know that, for an ideal gas

Density d = P × M/ RT

According to the question,

5.46 = 2 × M/ R × 300 ….. (1)

and, At STP, d = 1 × M/R × 273 …(2)

Dividing equation (ii) by (i)

d/5.46 = (1 × M/273 × R) × (300 × R/2 × M)

d = 300 × 5.46/ 273 × 2 = 3.00 g/dm^{3}

Hence, the density of the gas at STP will be 3 g dm^{-3}.