A gas sample has a volume of #x# #"dm"^3# at #20^@"C"#. If the pressure is halved, what temperature is required to maintain the volume at #x# #"dm"^3#?
1 Answer
Here's what I got.
Explanation:
The idea here is that when the number of moles and the volume of the gas are kept constant, pressure and temperature have a direct relationship as described by Gay Lussac's Law.
In other words, increasing the pressure by a factor will cause the volume to increase by the same factor. Likewise, decreasing the pressure by a factor will cause the volume to decrease by the same factor.
Mathematically, this is written as
#color(blue)(|bar(ul(color(white)(a/a)P_1/T_1 = P_2/T_2color(white)(a/a)|)))" "# , where
Rearrange the equation to solve for
#P_1/T_1 = P_2/T_2 implies T_2 = P_2/P_1 * T_1#
In your case, the volume of the gas must be kept constant at
#T_1 = 20^@"C" + 273.15 = "293.15 K"#
Now, the pressure is halved, which implies that
#P_2 = 1/2 * P_1#
Plug this into the equation to find
#T_2 = (1/2 * color(red)(cancel(color(black)(P_1))))/color(red)(cancel(color(black)(P_1))) * "293.15 K" = "146.6 K"#
Convert this back to degrees Celsius
#t_2 = "146.6 K" - 273.15 = color(green)(|bar(ul(color(white)(a/a)color(black)(-130^@"C")color(white)(a/a)|)))#
I'll leave the answer rounded to two sig figs.
So, when the pressure of a gas is halved, the only way to keep its volume constant is to halve its absolute temperature.