Question

how many moles of hcoona must be added to 1.0l of 0.1 M HCOOH to prepare a buffer solution with a pH of 3.4

Answer 1

Answer:- 0.045 moles of HCOONa are added.

Solution:- pH of buffer solutions is calculated using handerson equation:

$pH=Pka+log[\frac{base}{acid}]$

Here, the acid is formic acid that is HCOOH and its conjugate base is $HCOO^-$ which is given as the salt, HCOONa.

Moles of acid could be calculated from its given molarity and volume as:

moles = molarity * Liters

$1.0L(\frac{0.1mol}{L})$

= 0.1 mol

The pH is given as 3.4. Pka for formic acid could be found in the data table which is 3.75.

Let's plug in the values in the equation and solve for the moles of sodium formate.

$3.4=3.75+log[\frac{x}{0.1}]$

where $x$ is the moles of HCOONa.

$3.4-3.75=log[\frac{x}{0.1}]$

$-0.35=log[\frac{x}{0.1}]$

Taking antilog:

$10^-^0^.^3^5=[\frac{x}{0.1}]$

$0.45=[\frac{x}{0.1}]$

$x=0.45*0.1$

$x=0.045$

So, 0.045 moles of HCOONa are required.