Question

1. What is the pH of a buffer that is 0.177 M in a weak acid and 0.201 M in the acid's conjugate base? The acid's ionization constant is 5.7×10^-4

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2. A physiological solution contains 47 mEq/L of Cl^- ion. How many moles of Cl^- ion are present in 2.00 L of the solution?

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Answer 1

1.

pH of a buffer of weak acid and its conjugate base is given by he following equation

pH = pKa + log([A^-] / [HA]) -----1)

[HA] = concentration of weak acid ( = 0.177M), A- = concentration of conjugate base (= 0.201M)

pKa = -logKa

pKa = -log(5.7 X 10^-4) = 3.244

substituting the values in eqn 1)

pH = 3.244 + log ( 0.201 / 0.177 )

pH = 3.244 + 0.055 = 3.299

2.

Ionic mass of Cl^- = 35.45g

Therefore 1 Eq of Cl^- = 35.45g

One liter of physiological solution contains 47 mEq or 47 X 10^-3 Eq of Cl^- . Therefore grams of Cl^- are given as

47mEq = 47 X 10^-3 X 35.45g = 1.67g

moles of Cl^- in one liter = 1.67 / 35.45 = 0.047 Moles

Moles of Cl^- present in 2 L of the solution = 2 X 0.047 = 0.094 Moles