As an FDA physiologist, you need 0.700 L of formic acid- formate buffer with a p

Question

As an FDA physiologist, you need 0.700 L of formic acid- formate buffer with a pH of 3.74. a) What is the required buffer component concentration ratio? b) How do you prepare this solution from stock solutions of 1.0 M HCOOH and 1.0 M NaOH? c) What is the final concentration of HCOOH in this solution? A bioengineer preparing cells for cloning bathes a small piece of rat epithelial tissue in a TRIS buffer. The buffer is made by dissolving 43.0 g of TRIS (Pk_b = 5.91) in enough 0.095 M HCI to make 1.00 L of solution. What is the molarity of TRIS and the pH of the buffer?

Explanation / Answer

Q1.

V = 0.7 L

acid formate buffer

pH = 3.74, for the acid, pKa = 3.75

a)

find ratio of buffer

pH = pKa + log(formate / formic acid)

3.74 = 3.75 + log(formate/formic acid)

formate / formic acid = 10^(3.74-3.75) = 0.98 or approx 1, meaning

[ formate ] = [formic acid ]

b)

how to prepare from

1 M of acid, 1 M of base

so

we need 1:1 ratio, meaning that acid = formate

add the next:

0.700 / 3= 0.23333 L

add 0.23333*2 = 0.46666 L of acid solution

add 0.23333 L of base solution

mol of acid reacts with mol of base

ratio is now 2 mol of acid : 1 mol of base, after reaction

2 mol of acid - 1 mol of base = 1 mol of acid left

0 mol of conjugate + 1 mol of base = 1 mol of conjugate left

so this will have now:

1 : 1 ratio with respect to formic acid and formate

c)

final concentration of acid in this solution is:

0.46666*1 = 0.46666 mol of acid initially

0.23333*1 = 0.23333 mol of base initially

after reaction

0.23333 mol of acid left

0.23333 mol of conjguate formed

Final volume is 1 Liter so

M = mol/V = 0.23333/0.7 = 0.3333 M

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