Before planting her first tomato plants, Candy Chemist must prepare the soil for

Question

Before planting her first tomato plants, Candy Chemist must prepare the soil for the optimum pH, which for tomatoes, is about 6.40. In her chemical cabinet, Candy finds 5.00 L bottles labeled 1.50 M club soda, 1.50 protos pion, and 1.50 M caustic soda. From her general chemistry studies, she knows club soda is simply carbonic acid (H2CO3), which is a diprotic acid with Ka1= 4.22x10-11. From her Greek, biology and chemistry, she knows that protos pion is first-fat acid, which was the unnpleasent smell of body odor, is better known as propionic acid (C2H5COOH), and is a monoprotic acid with Ka= 1.34x10-5. Candy determine that 1.00 L of the appropriate buffer solution at pH 6.40 is required to treat the soil.

a.) Determine the best acid to prepare the buffer and determine the mole ratio of acid to conjugate base. Thinking of acids, just how many acids mentioned and available to Candy?

b.) Find the volume of 1.50 M caustic soda (NaOH) required to partially neutralize the appropriate acid and prepare 1.00 L of the buffer solution. You may assume all volumes are additive.

c.) What volume of 0.100 M NaOH must be added to 1.00 L of 0.500 M hypochlorous acid to achieve a pH of 8.00? (Ka(HOCl)=2.9x10-8)

Explanation / Answer

a) To get a buffer of pH 6.40, the acid we would use must have pKa close to this pH value.

Thus we would use H2CO3 (carbonic acid) with pKa 6.37 to prepare this buffer,

H2CO3 = acid

HCO3- = conjugate base

pH = pKa + log(conjugate base/acid)

(conjugate base/acid) = 6.40 - 6.37 = 1.07

So we would use a ratio of 1.07 for the preparation of buffer

c) Volume of NaOH needed to be added to HCOCl to get pH 8.0

let x amount of NaOH is added

pH = pKa + log(NaOCl/HOCl)

8 = 7.54 + log(x/(0.5 M x 1 L - x)

1.44 - 2.88x = x

x = 1.44/3.88 = 0.37 mol

moles of NaOH to be added = 0.37 mol

Volume of NaOH to be added = 0.37/0.1 = 3.7 L

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