Answers and Explanations

REVIEW QUESTIONS

1 . A   certain aqueous solution    at  25°C    has [OH−]   =   6.2 ×   10−5 M.

(A) Calculate   [H+].

(B) Calculate   the pH  of  the solution.

(C) Is  the solution    acidic  or  basic?

The concentration   of  H+  is  1.6 ×   10−10   M.  Kw =    [H+][OH−]   =   1.0 ×   10−14   M.  If  [OH−]   =   6.2 ×   10−5,

then    [H+]    =   Kw/[OH−]    =   1.6 ×   10−10 M.

A.

pH  =   −log    (1.6    ×   10−10)  =   9.79.

B.

C. A    pH  of  9.79    indicates   a   basic   solution.

2 . 1000:1

This    problem can be  solved  by  calculating the [H+]    of  the pH  =   4   solution    and the [H+]    of  the pH

=   7   solution.   Then    divide  the former  by  the latter: Since   pH  =   −log[H+],   [H+]    =   antilog (−pH).  For

pH  =   4,  antilog (−4)    =   10−4.   For pH  =   7,  antilog (−7)    =   10−7.   10−4:   1   ×   10−7    =   1000:1. Alternatively,

we  could   subtract    the pH’s    first,  and then    take    the antilog:

7   −   4   =   3   implies 10^3 ,  or  1000:1.

3 .  LiOH   →   Li+ +   OH−

A.

Ba(OH) 2    →   Ba2+    +   2OH−

B.