Question

A sample of gas has an initial volume of 25 L and an initial pressure of 3.5 Pa. If the pressure changes to 1.3 Pa, find the new volume, assuming that the temperature remains constant.

Answer 1

Since you are provided with the pressure and initial volume and told a standard temperature, you should use the Combined gas law. This is (P1 V1)/ T1 = (P2 V2)/ T2 
STP is 273 k for temperature, so plug in the values. 
(3.5 Pa * 25 L)/ 273 K =1.3 Pa * X L)/ 273 K.
(this is the left side>)0.32051= 1.3 X/ 273.  Multiply both sides by 273 to get rid of it on the right, which leaves you with
87.4999 = 1.3 X. Divide both sides by 1.3 and there you go, 

67.308 L