1. The isotope 8 He has a half life of 0.1225 s. HOw long will it take for the r

Question

1. The isotope 8 He has a half life of 0.1225 s. HOw long will it take for the radioactivity of a sample of this isotope to fall to 1/16 of its original intensity?

a. 0.25 s

b. 0.49 s

c. 0.37 s

d. 16 s

2. When a reaction shows one arrow pointing from left to right this means

a. there is practically no reactant left

b. the reaction has reached equilibrium

c. the reaction favors the right direction

d. both b and c are true

3.Consider the equation O2 (g) + 2H2 (g) <--> 2H2O (g). Increasing the concentration of hydrogen gas --

a. will shift the reaction to the left

b. will shfit the reaction to the right

c. will have no effect

d. cannot be determined, since the temperature is unknown

Explanation / Answer

1) answer : b. 0.49 s

Explanation :

t1/2 = 0.1225s

1/16 original intensity

we know ration ration of initial and remaining will give 1/2^n

N/N0 = 1/2n

1/16 = 1/2n

1/24 = 1/2n

n= 4  

time = number of half -lives x t1/2

t= n x t1/2

   = 4 x 0.1225

    = 0.49sec

2) answer: a. there is practically no reactant left

Explanation:

A+ B--------------------------> C + D

this means this reaction goes to completion at some time and total reactants converts into products so there is no reactant left

A+ B <--------------------------> C + D

it is equilibrium reaction this reaction never goes on completion . but one side may be favoured based on the conditions

3) answer : b. will shfit the reaction to the right

Explanation:

1) if concentration of reactants increases reaction shifts towards forward direction, more products will be formed and reaction shifted from left to right.

2) if concentration of products increases reaction shifts towards backward direction, more reactants will be formed and reaction shifted   from right to left .

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