A first order reaction has a specific reaction rate of 10^-2s^-1. How much time will it take for 20 g of the reactant to reduce to 5 g?

(a) 238.6 s      (b) 138.6 s        (c) 346.5  s        (d) 693.0 s

The decomposition of phosphine (PH₃) on tungsten at low pressure is a first-order reaction. It is because the

(1) rate is proportional to the surface coverage

(2) rate is inversely proportional to the surface coverage

(3) rate is independent of the surface coverage

(4) rate of decomposition is very slow

The rate of a first-order reaction is 0.04 mol L^-1 s^-1 at 10 sec and 0.03 mol L^-1 s^-1 at 20 sec after initiation of the reaction. The half-life period of the reaction is

1. 34.1 s

2. 44.1 s

3. 54.1 s

4. 24.7 s