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.

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!!