Abuffer consists of 0.21 m nah2po4 and 0.34 m na2hpo4. phosphoric acid is a triprotic acid (ka1 = 7.2 × 10−3, ka2 = 6.3 × 10−8, ka3 = 4.2 × 10−13). (a) which ka value is most important to this buffer? ka1 is most important. ka2 is most important. ka3 is most important. (b) what is the buffer ph?

a) Ka2 is the most important

b) The pH of the buffer is 7.41

Explanation:

Step 1: Data given

Molarity of NaH2PO4 = 0.21 M

Molarity of Na2HPO4 = 0.34 M

a) Which Ka value is most important to this buffer?

Phosphoric acid will ionize in aqueous solution according to the following equilibrium reactions

H3PO4(aq) + H2O(l) → H2PO4-(aq) + H3O+(aq)     Ka1 = 7.2*10^-3

H2PO4-(aq) + H2O(l) → HPO42-(aq) + H3O+(aq)     Ka2 = 6.3 *10^-8

HPO42-(aq) + H2O(l) → PO43-(aq) + H3O+(aq)     Ka3 = 4.2 * 10^-13

Ka2 is the most important constant, since it involves the equilibrium between the 2 phosporic acid salt species you have.   (H2PO4- and HPO42-)

b) What is the buffer pH?

H2PO4-(aq) + H2O(l) → HPO42-(aq) + H3O+(aq)

H2PO4-(aq) → HPO42-(aq) + H+

pH = pKa + log [conjugate base]/[acid]

pH = -log(6.3 * 10^-8) + log(0.34/0.21)

pH = 7.2 + 0.21

pH = 7.41

The pH of the buffer is 7.41

definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.