Sarthaks Test
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a). Rate constant k for first order reaction has been found to be 2.54 x 10‐3 s‐1 . Calculate its three fourth life. 

b). A reaction is first order in A and second order in B. 

(i) Write the differential rate equation. 

(ii) How is the rate affected on increasing the concentration of B three times? 

(iii) How is the rate affected when the concentrations of both A and B are doubled?

1 Answer

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(a) k = \(\frac{2.303}{t}\) log \(\frac{[R]_o}{[R]}\) 

For three-forth of the reaction to take place,

[R] = \(\frac{[R]_o}{[4]}\) or \(\frac{[R]_o}{[R]}\) = 4

Substituting the vale in the rate equation, we have

2.54 x 10-3\(\frac{2.303}{t}\)log 4

or t = \(\frac{2.303}{2.54}\) x 103 x 0.6021 = 5.46 x 102s

b) (i) dx/dt = k [A][B]2 

(ii) 9 times 

(iii) 8 times

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