When liquefied petroleum gas is sold at retail or wholesale or offered for sale at retail or wholesale by weight, in packages or containers, the tare weight of the container, and the net weight of the contents shall be plainly and conspicuously marked on the outside of the container or on a label firmly attached thereto. Tare weight shall not be construed to include the valve protecting cap, which shall be removed when weighing. It shall be a violation of sections 57-508 to 57-516 to sell or offer or expose for sale liquefied petroleum gas in packages or containers which do not bear a statement as to tare and net weight as required by this section, or which packages or containers bear a false statement as to weights.