Answers and Explanations

REVIEW QUESTIONS

1 . B

Radioactivity   is  a   first-order process.    After   three   half-lives, the concentration   of  the sample  will

be   ,  or   ,  of  the original.

2 . D

In  order   for a   64  g   sample  to  decay   to  one-eighth  of  its original    activity,   or  8   g,  the sample

would   have    to  go  through three   half-lives. Therefore,  the amount  of  time    needed  for the decay

is  3   ×   15  =   45  hours.

3 . First,  the general rate    equation    must    be  written out.    It  is:

rate    =   k[A]x[B]y

where:

k   = the   rate    constant

x   = the   order   with    respect to  reactant    A

y   = the   order   with    respect to  reactant    B

x   =   2

To  solve   for x,  it  is  necessary   to  find    two trials  in  which   B   is  held    constant;   here,

experiments 1   and 3.  The data    shows   that    if  the concentration   of  A   is  doubled,    the rate

increases   by  a   factor  of  4.  Thus,   the rate    varies  as  the square  of  the concentration   of  A.

The order   is  therefore   equal   to  2.

A.

B. y    =   0.5