At 300 k the pressure inside a gas filled container is 500 kpa if the pressure inside decreses to 100kpa but the volume remains the same what is the final temperature of the gas

Answer is: final temperature of the gas is 60 K.
Gay-Lussac's Law states that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature.
p₁/T₁ = p₂/T₂.
p₁ = 500kPa.
T₁ = 300 K.
p₂ = 100 kPa.
500 kPa/300K = 100 kPa/T₂.
T₂ = 300K · 100kPa ÷ 500 kPa.
T₂ = 60 K.

The mass has escaped into a gas which you can not see so the law stays true.