Question

What mass of Na3PO4 must be added to 80.0 mL of 0.20 M HCl to obtain a buffer with a pH of 7.75?

Answer 1

Answer:

Explanations:

First we need to determine the moles of base using the formula;

$pH=pKa+log(\frac{base}{Acid})$

where:

• pKa of Na3PO4 = 1.8

,

• pH =7.75

Determine the mole of the acid

$\begin{gathered} mole\text{ of HCl}=molarity\times volume \\ mole\text{ of HCl}=0.20M\times0.08 \\ mole\text{ of HCl}=0.016moles \end{gathered}$

Substitute the given parameters

$\begin{gathered} 7.75=1.8+ln(\frac{base}{0.016}) \\ 7.75-1.8=ln(\frac{base}{0.016}) \\ ln(\frac{base}{0.016})=5.95 \\ \frac{base}{0.016}=e^{5.95} \\ \end{gathered}$

From the calculation above, the base is calculated as

Base = 0.016e^5.95

Base = 6.14 moles

Determine the mass of the compound

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