Answer:
The pH value of the mixture will be 7.00
Explanation:
Mono and disodium hydrogen phosphate mixture act as a buffer to maintain pH value around 7. Henderson–Hasselbalch equation is used to determine the pH value of a buffer mixture, which is mathematically expressed as,
[tex]pH=pK_{a} + log(\frac{[Base]}{[Acid]})[/tex]
According to the given conditions, the equation will become as follow
[tex]pH=pK_{a} + log(\frac{[Na_{2}HPO_{4} ]}{[NaH_{2}PO_{4}]})[/tex]
The base and acid are assigned by observing the pKa values of both the compounds; smaller value means more acidic. NaH₂PO₄ has a pKa value of 6.86, while Na₂HPO₄ has a pKa value of 12.32 (not given, but it's a constant). Another more easy way is to the count the acidic hydrogen in the molecular formula; the compound with more acidic hydrogens will be assigned acidic and vice versa.
Placing all the given data we obtain,
[tex]pH=6.86 + log(\frac{0.058}{0.042})[/tex]
[tex]pH=7.00[/tex]
The pH of the conjugate pair mixture can be found using Henderson
Hassel Balch equation, given that the pKa is known.
Reasons:
The compounds and their concentration in the mixture are;
0.042 M solution of NaH₂PO₄
0.058 M solution of Na₂HPO₄
The pKa of the phosphate = 6.86
Solution:
The compounds NaH₂PO₄, and Na₂HPO₄, together form a conjugate acid
base pair, and therefore can used as a buffer solution.
The Henderson Hassel Balch equation for finding the pH of the solution
form by mixing the given compounds solutions is presented as follows;
In the mixture, NaH₂PO₄ is the proton donor, while Na₂HPO₄ is the proton
acceptor, therefore, we have;
[Proton acceptor] = Concentration of Na₂HPO₄ = 0.058 M
[Proton donor] = Concentration of NaH₂PO₄ = 0.042 M
Which gives;
The pH of the mixture is 7.000, therefore, NaH₂PO₄/Na₂HPO₄ conjugate
pair is a very good buffer.
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