Acid-base theory

Titrations

Exercise 2

       

$10,0 \;mL$ $H_3PO_4$ $0,1\; M$ are titrated by $NaOH$ $0,1\; M$. Let's call $x$ the volume of $H Cl$ already added at a given moment. a)Theoretically establish the function $pH = f(x)$ .

$x = 0$: Weak acid: $pH = 1.56$ $0 \lt x \lt 10$: Buffer: $pH $ $=$ $ 2.12$ $ + $ $log\frac{x}{ 10-x}$ $x = 10$: Amphoteric species: $pH = 4.66$ $10 \lt x \lt 20$: Buffer: $pH $ $= $ $7.20$ $ + $ $log\frac{x-20}{ 20-x}$ $x = 20$: Amphoteric species: $pH = 9.75$ $20 \lt x \lt 30$: Buffer : $pH$ $ = $ $12.3$ $ + $ $log\frac{x-20}{ 30-x}$ $x = 30$: Weak base: $pH = 12.45$

b)Give a table of values of the function by searching images $(1\;mL$ intervals of $x)$. Graph on graph paper please!

(Gray background: discrepancies due to the approximate approach!)

c) Choose a suitable indicators for this titration.

First transition : Bromocresol green. Second transition: Phenolphtalein.

d) Search graphically in what time interval during addition (between which x) we observe the color change of phenolphthalein. if that was the indicator chosen. Is this a suitable indicator?

Interval: $19.5 \lt x \lt 20.5$ OK!!