Acid-base theory

Titrations

Exercise 3

   

$10.0 cm3$ of an aqueous solution of methanoic acid are neutralized gradually by a decimolar a aqueous solution of sodium hydroxide. Here the experimental results:

Draw the graph $pH = f (x)$ on graph paper in order to deduce from this diagram

a) an approximate value of the formic acid molarity of the initial solution .

$c_{HCOOH}$ $=$ $0.10 M$.

b) an approximate value of the $pKa$ of formic acid.

$pKa$ $=$ $3.70$

c) the different species present in the solution for $x=6.0\; cm^3$ and their molarity.

It appears on the graph: $pH$ $=$ $3.9$ $[H_3O^+]$ $=$ $1.26\cdot 10^{-4}$. Let be $y$ the number of moles of $HCOO^-$: Use $3.9$ $=$ $3.7$ $+$ $log\frac{y}{0.001-y}$ to find $y$, then calculate the molarities: $[HCOOH]$ $=$ $2.4\cdot 10^{-2} M$ . $[HCOO^-]$ $=$ $3.8\cdot 10^{-2} M$. $[Na^+]$ $=$ $3.75\cdot 10^{-2} M$ . $[OH^-]$ $=$ $7.9\cdot 10{-11}$