Answer to Question #271446 in Physical Chemistry for Bhanu

Question

1.ph meter gives an electrical response proportional to the concentration of hydrogen ion. to convert this

response into ph, glass electrode must be calibrated against standard solution of known h+

concentration.

which of the following standard solution(s) is/are likely to be used?

a) ph 4.0 buffer b) ph 7.0 buffer c) ph 14.0 buffer and d) all of the above

Answer 1

pH of solution needed = 7.00

pKa of H2PO4- = 7.20

we have,

(H2PO4-) + (HPO4^2-) = 0.100 M x 1 L = 0.100 mol

Using Hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

7.00 = 7.20 + log(HPO4^2-/H2PO4-)

(HPO4^2-) = 0.631(H2PO4-)

feeding in the above equation,

(H2PO4-) + 0.631(H2PO4-) = 0.100 mol

(H2PO4-) mol = 0.100/1.631 = 0.061 mol

mass NaH2PO4.H2O needed = 0.061 mol x 138.01 g/mol = 8.5 g

(HPO4^2-) mol = 0.100 - 0.061 = 0.039 mol

mass Na2HPO4 needed = 0.039 mol x 141.98 g/mol = 5.5