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

Explanations:

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

`pH=pKa+log((base)/(Acid))`

where:

• pKa of Na3PO4 = 1.8

,

• pH =7.75

Determine the mole of the acid

`\begin{gathered} mole\text{ of HCl}=molarity* volume \\ mole\text{ of HCl}=0.20M*0.08 \\ mole\text{ of HCl}=0.016moles \end{gathered}`

Substitute the given parameters

`\begin{gathered} 7.75=1.8+ln((base)/(0.016)) \\ 7.75-1.8=ln((base)/(0.016)) \\ ln((base)/(0.016))=5.95 \\ (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