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In: Chemistry

Use the Henderson-Hasselbalch equation to calculate the pH of each solution: 1)a solution that is 0.170...

Use the Henderson-Hasselbalch equation to calculate the pH of each solution:

1)a solution that is 0.170 M in HC2H3O2 and 0.115 M in KC2H3O2

2) a solution that is 0.230 M in CH3NH2 and 0.130 M in CH3NH3Br

Solutions

Expert Solution

For your reference,

Henderson-Hasselbalch equation is :

pH = pKa + log([Proton Acceptor]/[Proton Donor])

or

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

Now,

Solution 1 :

CH3COOK gives CH3COO- which is A- in this case.

and CH3COOH is HA.

[A-] = 0.115 M

[HA] = 0.170 M

NOTE: pKa should either be given in the question or Ka should be given so that you can find pKa by : pKa = -log(Ka)

pKa for acetic acid is 4.74

Now,

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

pH = 4.74 + log(0.115/0.170) = 4.74 - 0.17 = 4.57

Solution 2 :

[A-] = [CH3NH3Br] = 0.130 M

[HA] = [CH3NH2] = 0.230 M

NOTE: pKa should either be given in the question or Ka should be given so that you can find pKa by : pKa = -log(Ka)

pKa for CH3NH2 is 3.36

Now,

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

pH = 3.36 + log(0.130/0.230) = 3.36 - 0.25 = 3.11

Answer might be a little different since it might have been calculated with an approximated value of pKa

Thanks. Comment if any doubt.


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