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Which acid would you use to make a buffer with pH = 7.35?

If you have 500.0 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?

Chlorous acid: pKa = 1.95
Formic acid: pKa = 3.74
Nitrous acid: pKa = 3.34
Hypochlorous acid: pKa = 7.54

1 Answer

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Final answer:

To make a buffer with a pH of 7.35, you would use Hypochlorous acid. The mass of the corresponding sodium salt of the conjugate base will depend on the specific acid used, and can be calculated using the Henderson-Hasselbalch equation.

Step-by-step explanation:

To make a buffer with a pH of 7.35, we need to choose an acid with a pKa close to that value. From the provided options, the acid with the closest pKa is Hypochlorous acid (pKa = 7.54). Therefore, we would use Hypochlorous acid to make the buffer.

To calculate the mass of the corresponding sodium salt of the conjugate base, we can use the Henderson-Hasselbalch equation:

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

Since we have a 0.10 M solution of the acid, the concentration of HA is 0.10 M. We can rearrange the equation to solve for [A-]:

[A-] = 10^(pH - pKa) * [HA]

Substituting the values:

[A-] = 10^(7.35 - 7.54) * 0.10 M

[A-] = 0.0666 M

The molar mass of the corresponding sodium salt of the conjugate base will depend on the specific acid used. For example, if the acid is chlorous acid (HClO2), the corresponding sodium salt would be sodium chlorite (NaClO2). To calculate the mass of the salt needed, we can use the formula:

mass = molar mass * volume * concentration

Assuming a density of 1 g/mL, we can calculate the required mass:

mass = molar mass * 500.0 mL * 0.0666 M

Remember to adjust the molar mass according to the specific acid used.

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