Question

A reaction is 50% completed in 2 hours and 75% completed
in 4 hours. The order of reaction is [NEET Kar. 2013]
(a) 0 (b) 1
(c) 2 (d) 3

Answer 1

Answer : option (b) first order reaction,

here it is given that, 50% completed in 2 hours.

so, t½ = 2 hours

but 75% completed in 4 hours.

means, t¾ = 4 hours

here we see that, t¾ = 2t½ this is true only for first order reaction. hence option (b) is correct choice.

[ for first order reaction, t = 1/k lna/(a - x), for when x = 50% of a then, t½ = 1/k lna/(a/2) = ln2/k [constant ], when x = 75% of x, t¾ = 1/k a/(a/4) = ln4/k = 2ln2/k = 2t½ hence, t¾ = 2t½ . ]

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