Question

$NaOH (a q), HCl (a q), $ and $ NaCl (a q)$ concentration of each is $10^{-3} \,M$. Their $pH$ will be respectively :

• A. 10, 6, 2
• B. 11, 3, 7
• C. 10, 3, 7
• D. 10, 4, 7

Answer 1

Answer: (B)

Use $p H=-\log \left[ H ^{+}\right]$
and $p H+p O H=14$ to solve problem.
$pH$ of $1 0 ^{-3} M\, NaOH$
$\underset{10^{-3}\,M}{NaO H} \longrightarrow Na ^{+}+ \underset{10^{-3}\,M}{OH ^{-}} $
$ pOH =-\log 10^{-3} $
$=3 $
$ pH + pOH =14 $
$pH =14-3=11$
pH of $10^{-3} HCl$
$ \underset{10^{-3}\,M}{HCl} \longrightarrow \underset{10^{-3}\,M}{H ^{+}}+ Cl ^{-} $
$pH =-\log \left[ H ^{+}\right]$
$=-\log \left[10^{-3}\right]$
$=3$
if of $10^{-3} NaCl$ is $7$ (neutral)
$\because$ It is salt of strong acid and strong base.