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A buffer, consisting ofH2PO4-
andHPO42-, helps control the pH of
physiologicalfluids. Many carbonated soft drinks also use this
buffer system.What is the pH of a soft drink in which the major
bufferingredients are 6.70 g ofNaH2PO4 and
6.50g of Na2HPO4 per 355 mL
ofsolution?

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Answer:

The pH of the drink is 7.12

Explanation:

First, we calculate the concentration of NaH₂PO₄ and Na₂HPO₄, using their molecular weight and the volume in L (355 mL= 0.355 L):

[NaH₂PO₄] = [tex]\frac{6.70g}{0.355L*120g/mol}= 0.1573 M[/tex]

[Na₂HPO₄] = [tex]\frac{6.50g}{0.355L*142g/mol} = 0.1289 M[/tex]

Now we calculate the pH of the solution, keeping in mind the equilibrium:

  • H₂PO₄⁻ ↔ HPO₄⁻² + H⁺  

From literature, we know that the pka for the previous equilibrium is 7.21

The equation that gives us the pH of a buffer solution is the Henderson–Hasselbalch equation:

pH = pka + [tex]log\frac{[HPO4^{-2} ]}{[H2PO4^{-} ]}[/tex]

Replacing in the equation the data we know gives us:

[tex]pH=7.21+log\frac{0.1289M}{0.1573M} \\pH=7.12[/tex]

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