A mixture of 4g of hydrogen and 8 g of helium at NPT has a density about
- $0.22kg/m^{3}$
- $0.62kg/m^{3}$
- $1.12kg/m^{3}$
- $0.13kg/m^{3}$
At NPT, 1 mole of any gas occupies 22.4 L. Hydrogen (H2): 4g = 2 moles. Helium (He): 8g = 2 moles. Total moles = 4. Total volume = 4 * 22.4 L = 89.6 L = 0.0896 m^3. Total mass = 4g + 8g = 12g = 0.012 kg. Density = 0.012 / 0.0896 = 0.1339 kg/m^3.
The number of moles of hydrogen is 4 g divided by 2 g/mol, yielding 2 moles, while the moles of helium are 8 g divided by 4 g/mol, giving 2 moles. The total moles in the mixture are therefore 4 moles, and at normal temperature and pressure, this gas occupies 4 moles multiplied by 22.4 L/mol, or 89.6 litres. The total mass of the mixture is 12 g, which equals 0.012 kg, and dividing this by the volume in cubic metres (0.0896) gives a density of approximately 0.13 kg/m^3.