I have an exam tommorow ( chemistry)

Question

1/ A sample of neon initially has a volume of 2.50 L at 15 degrees Celsius. What is final temperature, in degrees Celsius, is needed to change the volume of the gas to 1.25 L?

2/A 10.0 L balloon contains helium gas at a pressure of 655 mmHg. What is the final pressure, in millimeters of mercury, if the final volume is 13.8 L? 

Answer 1

1/

the equation to use here is from Charles's law: T2 / V2 = T1 / V1 with T being in kelvin. therefore, T2 = T1 * V2 / V1. T1 = 15 + 273.₁₅ = 288.₁₅ K, V1 = 2.50 L and V2 = 1.25 L. substituting the values in the above equation for T2 and simplifying, one gets T2 = 144 K = 144 - 273 = - 129 deg C. the volume is halved and so the temperature should also be halved. but, you should halve the kelvin temperature, not the deg C temperature

2/

here the equation to use is from Boyle's law: P2 V2 = P1 V1. therefore, P2 = P1 V1 / V2. P1 = 655 mm Hg, V1 = 10.0 L and V2 = 13.8 L. substituting the values in the above equation for P2 and simplifying, one gets P2 = 475 mm Hg. when the volume is increased, the pressure should decrease. the result is consistent with this expectation