In the previous blog post, we have discussed on the The Gaseous State for A-Level H2 Chemistry and the meaning of Ideal Gas (versus a Real Gas).
We have also discussed on Ideal Gas Equation and how to apply it on exam-based question.
Today, we shall look at another common exam-based question that has been seen modified versions in many Junior Colleges (JC) and Institutes in Singapore.
At 338K, pure PCl5 gas present in a flask has a pressure of 25.5 kPa. At 480K, the gas is completely dissociated into PCl3(g) and Cl2(g). What is the pressure in the flask at 48oK?
From the Ideal Gas Equation of PV = nRT,
we have P = (nRT)/V
Since V and R are constants,
we have (n1T1)/P1 = (n2T2)/P2
The next step is to write a balanced chemical equation to show the dissociate of PCl5 gas at 480K, which is:
PCl5(g) –> PCl3(g) + Cl2(g)
As such, the number of moles of gases has doubled at 480K since 1 mol of PCl5 has dissociated to produce 2 moles of gases (i.e. 1 mol of PCl3 and 1 mol of Cl2).
Thus, ratio of n2:n1 is 2:1
From (n1T1)/P1 = (n2T2)/P2,
we have P2 = (n2T2)(P1)/(n1T1) = (2 x 480)(25.5)/(1 x 338) = 72.4 kPa
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