Question

2) What volume of 50 mM Na2HPO4 and 100 mM NaH2PO4 is needed to prepare 3L of 25 mM
phosphate buffer, pH 6.5 (pKa = 7.21)?

Answer 1

Using Henderson equition

$PH=pKa+log\frac{A^-}{HA}\\6.5=7.21+log\frac{A^-}{HA}\\log\frac{A^-}{HA}=0.1950\\^-=0.1950(HA)$

Also

$(A^-)+(HA)=25mm\\0.1950(HA+(HA)=25mm\\(HA)=20.9208mm\\(A^-)=0.1950(HA)\\=4.0792$

Need: 3 L of 20.9206mm NaH₂PO₄

$M_1V_1=M_2V_2\\100mm×V=20.9206mm×3L\\V=0.6276L\\=627.624ml$

Need: 3 L of 4.0792mm NaH₂PO₄

Similarly

$50mm×V=4.0792mm×300ml\\=244.75mL$