Chemistry Exam Help?

Question

Well the AP exam is coming, and I would appreciate it if someone could help walk me through this question. I have an answer, but I ma not sure if it is correct.

A certain gas in a 2.75L container has a pressure of 875mm Hg at 25 degrees C. What will be the final temp of the sample if it is placed in a 750 mL container under 1.50 atm pressure?

Answer 1

Combined gas law:
P1V1T2 = P2V2T1

875 x 2.75 x T2 = 1140 x 0.75 x 298* : (K = 25 + 273)*

T2 = (1140 x 750 x 298) ÷ (875 x 2750)

T2 = 254,790 ÷ 2406.25

T2 = 105.89 K: = 105.89 - 273

Final temp. = -167°C

Answer 2

First of all, you want the pressure in Pascals, not mmHg.
There are 133.322 Pa to 1 mmHg. Therefore:
875 mmHg = 116656.75 Pa.

For a fixed quantity of gas, if it starts off at pressure, volume and temperatures, P(1), V(1) and T(1), and ends up with pressure volume and temperature, P(2), V(2) and T(2),
then:
(P(1)V(1))/T(1) = (P(2)V(2))/T(2).

Your final pressure is quoted in atmospheres. 1 atm = 101325 Pa, so 1.5 atm = 151987.5 Pa.

P(1) = 116656.75 Pa
V(1) = 2.75L = 0.00275 m^3
T(1) = 298K (Temperature needs to be in kelvins, not degrees celcius: Add 273 to you celcius value).
P(2) = 151987.5 Pa
V(2) = 0.00075 m^3

So, substitute in these numbers to the above equation. This gives you:

(116656.75 Pa x 0.00275 m^3)/298 K = (151987.5 Pa x 0.00075 m^3)/T(2)
Simplify this: 1.076 = 113.99/T(2)
Rearange: 113.99/1.076 = T(2) = 105.9 K

The point is, PV/RT is always constant for a fixed quantity of gas, and you must make sure you have the right units for all numbers: Pressure- Pa (N/m)
Volume- m^3
Temperature- K
The method above is correct, but I suggest you double check my workings, as I occasionally err.

Answer 3

The combines gas law says: P1V1/T1 = P2V2/T2

So you can plug in the numbers and solve for T2, but change mm to atm and T to K. (sorry, no calculator handy)