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In the first order gas phase reaction A(g) -> B(g) + C(g) followed by total pressure, pi is the initial pressure of A and pt the total pressure at time t. What is the partial pressure pA of A at time t?
Apt - pi
B2pi - pt
Cpi + pt
Dpt - 2pi
Answer & Solution
Correct answer: B. 2pi - pt
1. If x is the decrease in A, then B and C each rise by x, so $p_t = (p_i - x) + x + x = p_i + x$.
2. Hence $x = p_t - p_i$.
3. $p_A = p_i - x = p_i - (p_t - p_i) = 2p_i - p_t$.
4. Distractor A gives x itself, not pA; distractor C adds pressures incorrectly.
_Source: NCERT Class 12 Chemistry Ch 3 "Chemical Kinetics", p.14_
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