Show that the volume of 1.23 x 10^22 atoms of helium at standard laboratory conditions is equal to 0.49L.
Like the standard temperature and pressure set by the IUPAC, the standard laboratory condition (SLC) refers to the standard temperature and pressure values in a laboratory. The temperature and pressure is does not have a uniform value for all countries. In the US, the temperature at SLC is 25 degree Celsius and the pressure is one atmosphere; at this temperature and pressure the volume of one mole of a gas is equal to 24.5 liters.
One mole of a gas has 6.023*10^23 particles.
1.23*10^22 atoms of helium are equivalent to 1.23*10^22/6.023*10^23 mole = 0.0204 moles.
At SLC one mole of a gas occupies 24.5 l, 0.0204 mole of a gas occupies 0.0204*24.5 = 0.5 l.