pH of acids, bases and salts

Exercise 17

    

Calculate the mass of potassium hydoxide that must be dissolved in a volumetric flask $J$ of 250 $mL$ in order to obtain a solution with $pH$ = 12.7.

$KOH$: strong base Final solution: $pH$ $=$ $14$ $+$ $log\;[OH^-]$ $[OH^-]$ $=$ $10^{pH-14}$ = 0.05 $\frac{mol}{L}$ $c_{KOH}$ $=$ $\frac{0.05}{1}$ $n_{KOH}$ = $c_{KOH}\cdot V_{J}$ = 0.05$\cdot$0.25 = 0.0125 $\;mol$ $m_{KOH}$ = $n_{KOH}\cdot M_{KOH}$ = 0.0125$\cdot$56.11 = 0.701$\; g$