What would be the pH of a solution prepared by combining equal quantities of NaH2PO4 and Na2HPO4? Explain with an equation.
ka2=6.2x10^-8
pka2=-log(ka2)=-log(6.2x10^-8)=7.2
pH=7.2
It will be pKa2.
pH = pKa2 + log[(base)/(acid)]
If base = acid, then log 1 = 0 and pH = pKa2.
Look up Ka2, (and Ka1 and Ka3 will be in the same table) for H3PO4.
Then pKa2 = -log(Ka2) = ?
Well, let me do some clown calculations for you! When you combine NaH2PO4 and Na2HPO4, you get a buffer solution that can resist changes in pH. The dissociation reactions of these compounds are:
NaH2PO4 ⇌ Na+ + H2PO4-
Na2HPO4 ⇌ 2Na+ + HPO42-
Now, if we mix equal quantities of NaH2PO4 and Na2HPO4, we can write an equation for the buffer reaction:
NaH2PO4 + Na2HPO4 ⇌ Na+ + H2PO4- + 2Na+ + HPO42-
This can be simplified to:
H2PO4- ⇌ HPO42-
This reaction involves the hydrogen phosphate ion (HPO42-) and the dihydrogen phosphate ion (H2PO4-), which can act as a weak acid and its corresponding conjugate base. The pKa of this reaction is around 7.2.
So, based on this equation and the pKa value, the pH of the solution would be around 7.2 (with some clown margin for error!).
To determine the pH of the solution prepared by mixing NaH2PO4 (sodium dihydrogen phosphate) and Na2HPO4 (disodium hydrogen phosphate), we need to consider the acid-base properties of these compounds.
NaH2PO4 can act as a weak acid, while Na2HPO4 can act as a weak base. When these two compounds are combined in equal quantities, they can undergo a reaction known as a buffer reaction.
The buffer equation for this reaction is:
NaH2PO4 + Na2HPO4 ⇌ Na2HPO4 + NaH2PO4
In this equation, the Na2HPO4 and NaH2PO4 on either side represent the conjugate base and conjugate acid pairs, respectively. This buffer system is referred to as a phosphate buffer system.
The acid dissociation constant, Ka, can be represented as:
Ka = [H+][HPO42-] / [H2PO4-]
Since the solution is prepared by mixing equal amounts of NaH2PO4 and Na2HPO4, the concentrations of HPO42- and H2PO4- will also be equal. Thus, we can simplify the equation to:
Ka = [H+] / [HPO42-]
To calculate the pH, we can use the Henderson-Hasselbalch equation, which is applicable for buffer solutions:
pH = pKa + log ([A-] / [HA])
In our case, A- represents HPO42-, and HA represents H2PO4-.
In order to find the pH, we need to know the pKa value for the phosphate buffer system.