Question

If you have 500 mL of a 0.10 M solution of the acid, what mass of the corresponding sodium salt of the conjugate base do you need to make the buffer with a pH of 7.74 (assuming no change in volume)

Answer 1

Answer:

5.90g of NaClO are needed

Explanation:

Assuming the weak acid is HClO pKa = 7.54

To solve this question we must use the H-H equation for acids:

pH = pKa + log [A-] / [HA]

Where pH is the pH we want = 7.74

pKa is pKa of the weak acid = 7.54

[A-] could be taken as the moles of the conjugate base (Moles NaClO) and [HA] moles weak acid = 0.500L * (0.10mol/L) = 0.0500 moles HClO

Replacing:

7.74 = 7.54 + log [NaClO] / [0.0500 moles]

0.20 = log [NaClO] / [0.0500 moles]

1.5849 = [NaClO] / [0.0500 moles]

Moles NaClO = 0.0792 moles

The mass is: -Molar mass NaClO = 74.44g/mol-

0.0792 moles * (74.44g / 1mol) =

5.90g of NaClO are needed

Answer 2

5.90g of NaClO mass of the sodium salt of the conjugate base are need to make the buffer with a pH of 7.74.

What is pH?

pH tells the solution is acidic or basic. pH means potential hydrogen.

Given,

Assuming the weak acid is (hypochlorous acid) HClO

pKa = 7.54

$pH = pKa + \dfrac{ log[A-] }{ [HA]}$

(Moles NaClO) and [HA] moles weak acid = 0.500L * (0.10mol/L) = 0.0500 moles HClO

Putting the value

$7.74 = 7.54 + \dfrac{ log[NaClO] }{ [0.0500 moles]}\\0.20 =\dfrac{ log}{[0.0500 moles]} \\1.5849 =\dfrac{ [NaClO]}{Moles\;of\; NaClO } = 0.0792 moles$

The mass is

Molar mass NaClO

= 74.44g/mol - 0.0792 moles * (74.44g / 1mol)

=5.90g of NaClO are needed

Thus, 5.90g of NaClO are needed.

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