Question

To make 1.00 L of a 0.100 M H₂PO₄^- / HPO₄^2- buffer at pH 6.90 and you start with the NaH₂PO₄,

A. Calculate the mass of NaH₂PO₄ needed to make the buffer.

B. Should HCl or NaOH be added to make the final buffer? Calculate the number of moles of HCl or NaOH that needs to be added.   Note: formula mass of NaH₂PO₄ is 120.

Answer 1

Total moles of H2PO and HPO is the initi al moles of NaH,PO, mol NaH2PO4 = 1.00 L × 0.100 M = 0.100 mol Mass of NaH2PO, - molesxmolar mass -0.100 molx120 g/mol - 12.0g The equilibriums of H2PO Therefore, NaOH is added to make the buffer The number of moles of NaOH added is equal to the number of moles of HPO produced HPO HPO:- HPOİ 6.90 = 7.20 + log HPO lo H,POT HPO 10030.501 [HPO:] 0.501[HPOİ] . (2) From equations(1) and (2 (0.100-H,PO)0.501[H,POT] LHPGJ-0.066614 HPO0.100-(0.0666 0.0334 M moles HPO4-= 1.00 L x 0.0334 M = 0.0334 mol Therefore, the number of moles of NaOH added is 0.0334 mol