Final answer:
The density of helium at STP is 0.1786 g/L, which rounds to 0.179 g/L when expressed to three significant figures.
Step-by-step explanation:
The density of helium (He) at standard temperature and pressure (STP) is typically found using the molar mass of helium and the volume of one mole of an ideal gas at STP. Helium has a molar mass of approximately 4.00 g/mol. At STP, one mole of an ideal gas occupies 22.4 liters. To find the density, we divide the molar mass by the molar volume:
Density of He at STP = Molar mass / Molar volume
Density of He at STP = 4.00 g/mol / 22.4 L/mol
= 0.1786 g/L (when expressed to four significant figures)
However, given that the student is asking for a three significant figures answer, we would round this to 0.179 g/L.