Question

Phosphoric acid is a triprotic acid with the pK, values pK 1 = 2.148, PK 2 = 7.198, and pk 3 = 12.375. You wish to prepare 1.000 L of a 0.0100 M phosphate buffer at pH 7.720. To do this, you choose to mix the two salt forms involved in the second ionization, NaH,PO, and Na, HPO,, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? mass NaH,PO, = mass Na2HPO4 = What other combination of phosphoric acid and its salts could be mixed to prepare this buffer? Na,HPO, and Na, Po H,PO, and NaH,PO H,PO, and Na PO, NaH,PO, and Na PO, OH,PO, and Na2HPO.

Answer 1

Solution:

We have to prepare 1L of 0.01 M phosphate buffer solution at pH 7.720 by mixing two salt NaH2PO4 and Na2HPO4 in 1L volumteric flask.

(A) The concentration of Na2HPO4 and NaH2PO4 can be calculating by using Henderson-Hessalbalch equation.

pH= pka + log [A^{​​​​​​-}​​​​​]/[HA^{​​​​​-} ​​​​​​]

Where [A^{​​​​​​-}] = concentration of conjugate base

[HA^{​​​​​-}​​​​] = Concentration of an acid

We have to calculate at pH 7.720 therefore we are using pka is 7.198.

7.720 = 7.198 + log [A^{​​​​​​-}​​​​​]/[HA^-]

[A-]/[HA-] = 1.685

Hence the ratio of molarity of two salts should be around 1.685 M and the sum of the concentration of the acid and base should be equal to 0.01 M.

Therefore [A-]= 1.685*[HA-] ........(1)

[A-] + [HA-] = 0.01

[HA-] = 0.01 - [A-].......(2)

Put the value from equation (2) in (1)

[A-] = 1.685 * ( 0.01 - [A-])

[A-] = 0.01685/2.685

[A-] = 0.0063 M

Put the value of base in equation (2)

[HA-] = 0.01 - 0.0063

[HA-] = 0.0037 M

Hence we can prepare the solution using the concentration of Na2HPO4 and NaH2PO4 are 0.0063M and 0.0037 M.

The mass of Salt can be calculate using molarity formula:

M= w/m * 1/l

Where M = molarity , w = weight , m= molecular weight and l = volume in l

Mass of Na2HPO4, W1 = M * m * l

W1= 0.0063 * 268.07 * 1

W1 = 1.69 g

Mass of NaH2PO4, W2= 0.0037 * 137.99* 1

W2 = 0.51 g

(B) The other combination of phosphuric acids and it's salts

the correct example are H3PO4 and Na3PO4 , NaH2PO4 and Na3PO4 , H3PO4 and Na2HPO4