pH of acids, bases and salts

Exercise 12

    

Using the table of acid-base couples, calculate the $pH$ of a 2.5 %  ($d$ = 1.0242)  sodium carbonate solution named $S$

$CO_3^{2-}$: weak base If $d$ = 1.0242, then $\rho$ = 1.0242 $\frac{g}{mL}$ Let's take $1\;L$ of the solution $S$: $m_S$ $=$ $\rho \cdot V_S$ $ =$ $ \rho \cdot 1000 $ = 1024.2$\; g$ $m_{Na_2CO_3}$ = $\frac{\%_{Na_2CO_3}\cdot m_S}{100} $ = $\frac{2.5 \cdot1024.2}{100} $ = 25.61$\; g$ $n_{Na_2CO_3}$ $=$ $\frac{m_{Na_2CO_3}}{M_{Na_2CO_3}}$ = $\frac{25.61}{106}$ = 0.242$ \;mol$ $c_{Na_2CO_3}$ = $\frac{n_{Na_2CO_3}}{V_S}$ = $\frac{0.242}{1}$ = 0.242 $\frac{mol}{L}$ $c_{Na_2CO_3}$ $=$ $c_{CO_3^{2-}}=c$ Given $z=[OH^-]$ The equation $z^2$ $+$ $K_bz$ $-$ $K_bc$ $=$ $0$ becomes: $z^2$ $+$ $10^{-3.68}z$ $-$ $10^{-3.68} 0.242$ $= $ $0$ and produces: $z$ $=$ 7 10^-3 and so: $pOH$ $=$ $-log\; z $ = 2.15 $pH$ $=$ $14$ $-$ $pOH$ = 11.85