A certain volume of a gas had a pressure of 800 torr at a temperature of -40 degrees C. What was the original volume if the volume at STP is now 450.0 cm^3? (the correct answer is 365 cm^3. I just need an explanation.)

Answer : The original volume of gas is

Explanation :

Combined gas law is the combination of Boyle's law, Charles's law and Gay-Lussac's law.

The combined gas equation is,

where,

= initial pressure of gas = 800 torr

= final pressure of gas at STP = 760 torr

= initial volume of gas = ?

= final volume of gas at STP =

= initial temperature of gas =

= final temperature of gas at STP =

Now put all the given values in the above equation, we get:

Therefore, the original volume of gas is