Answer
a) pH = 4.70
b) pH = 11.6
c) pH = 8.449
d) pH = 3
e) pH = 15.08
Work Step by Step
- The rule for significant figures for the pH scale works like this:
The number of significant figures of the $[H_3O^+]$ in M, has to be the same numbers $after$ the "." in pH scale.
For example:
$[H_3O^+] = 3.56 \times 10^{-9}M$, there are 3 significant figures.
- The number that appears in the calculator; for the pH, is:
pH = 8.448550....
And , we have to have 3 significant figures after the ".", so:
pH = 8.449
a)
$pH = -log[H_3O^+]$
$pH = -log( 2 \times 10^{- 5})$
$pH = 4.699 = 4.70$
b)
$pOH = -log[OH^-]$
$pOH = -log( 4 \times 10^{- 3})$
$pOH = 2.398$
$pH + pOH = 14$
$pH + 2.398 = 14$
$pH = 11.602 = 11.6$
c)
$pH = -log[H_3O^+]$
$pH = -log( 3.56 \times 10^{- 9})$
$pH = 8.449$
d)
$pH = -log[H_3O^+]$
$pH = -log( 1 \times 10^{- 3})$
$pH = 3$
e)
$pOH = -log[OH^-]$
$pOH = -log( 12)$
$pOH = -1.079$
$pH + pOH = 14$
$pH + -1.079 = 14$
$pH = 15.079 = 15.08 $
