> This is highly dependent on temperature and pressure. Probably because they are talking about liquefied helium. A convention to reserve mass as the unit of

> This is highly dependent on temperature and pressure. Probably because they are talking about liquefied helium. A convention to reserve mass as the unit of sale for elements in gaseous form seems reasonable, you would want to receive a Very Large Bladder of gas when ordering supplies for an experiment that needs to be chilled with helium.