By making a titration of $ 10 \; mL\; NH_3 $ with $ HCl \; 0.01 \; M $, thymol blue (turning range 1.2-2.8) begins to turn approximately after adding $ 22 \; mL \;HCl $! In what part of the titration curve does this occur?
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Probably in the region after E.P. where acidity is high.
Find the number of $ HCl $ moles in excess at this time!
$pH=-log\;c_{HCl}$ Turn begins at $pH=2.8$: $2.8=-log\frac{n_{HCl}}{0.010+0.022}$ $n_{HCl}=5.1\;10^{-5}mol$
Find the initial number of moles of $ NH_3 $ !
The initial number of moles of $ NH_3 $ = Number of moles $ HCl $ added at E.P. = Number of moles $ HCl $ added at turn - number of moles $ HCl $ in excess = $0.022\cdot 0.01-5.1\;10^{-5}$ $=$ $1.7\cdot 10^{-4}\;mol$
Trouver la molarité initiale $c_b$ de $NH_3$ (Remarque: Résultat très approximatif. Mieux vaut prendre un indicateur qui vire approximativement au p.e!) $c_b$ $=$ $\frac{1,7\cdot 10^{-4}}{0,010}$ $=$ $1,7\cdot 10^{-2}M$
