Answer:- 1.35 grams of NaF must be added.
Solution:- HF is a weak acid. So, if NaF is added to it then a buffer solution will be formed and the Handerson equation is used for buffer solutions to find out the pH. Since the pH is given, so we could easily calculate the moles and then grams of NaF required to achieve the mentioned pH.
Handerson equation is:
[tex]pH=pKa+log(\frac{base}{acid})[/tex]
Moles of HF could be calculated from it's given volume and molarity as:
[tex]50.0mL(\frac{1L}{1000mL})(\frac{0.500mol}{1L})[/tex]
= 0.025 mol HF
pKa is calculated from given Ka value using the formula:
[tex]pKa=-logka[/tex]
[tex]pKa=-log7.2*10^-^4[/tex]
pKa = 3.14
Let's plug in the values in the Handerson equation:
[tex]3.25=3.14+log(\frac{base}{0.025mol})[/tex]
[tex]3.25-3.14=log(\frac{base}{0.025mol})[/tex]
[tex]0.11=log(\frac{base}{0.025mol})[/tex]
On taking antilog:
[tex]10^0^.^1^1=(\frac{base}{0.025mol})[/tex]
[tex]1.29=(\frac{base}{0.025mol})[/tex]
[tex]base=1.29*0.025mol[/tex]
base = 0.03225 mol
Molar mass of NaF is 41.99 grams per mol. Let's multiply the moles by molar mass to get the grams of NaF.
[tex]0.03225mol(\frac{41.99g}{1mol})[/tex]
= 1.35 g
So, 1.35 grams of NaF must be added to 50.0 mL of 0.500 M HF solution to achieve a pH of 3.25.
