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.
Question:
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?
Suggested Answer:
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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