Bank Soal Kimia SMA Derajat Keasaman (pH)

Diketahui:

V = 1 liter

pH = 2 $-$ log 3,3

$K_{\text{a}}=1,1\times10^{-2}\$

$A_{\text{r}}:\ \text{H}=1,\ \text{Cl}=35,5,\ \text{O}=16\ \$

Ditanya:

Berapa massa dari HNO₂?

Jawab:

$\text{pH}=-\ \log\left[\text{H}^+\right]$

$2-\log\ 3,3=-\ \log\left[\text{H}^+\right]\ \ \$

$\left[\text{H}^+\right]=\ 3,3\times10^{-2}$

$\left[\text{H}^+\right]\ =\ \sqrt{K_{\text{a}}\ \times M_{\text{a}}\ }\ \ \$

$3,3\ \times10^{-2}\ =\ \sqrt{1,1\ \times10^{-2}\ \ \times M_{\text{a}}\ }\ \ \ \$

$\left[3,3\ \times10^{-2}\right]^2=\left[\sqrt{1,1\ \times10^{-2}\ \ \times M_{\text{a}}}\right]^2$

$10,89\ \times10^{-4}\ =\ 1,1\ \times10^{-2}\ \ \times M_{\text{a}}\ \ \$

$M_{\text{a}}=\frac{10,89\ \times10^{-4}\ }{1,1\ \times\ 10^{-2}}$

$M_{\text{a}}=9,9\ \times10^{-2}\$

$M_{\text{a}}=0,099\ =\ 0,1\$

$M\ =\frac{\text{massa}}{M_{\text{r}}}\times\frac{1.000}{V\left(\text{ml}\right)}$

$0,1=\frac{\text{massa}}{68,5}\times\frac{1.000}{1.000}$

$\text{massa}=\ 0,1\ \times68,5$

$\text{massa}=\ 6,85\ \text{gram}\ \ \$

Jadi, massa HClO₂ adalah 6,85 gram.