Question 3) A 1.00 L buffer solution is 0.250 M in HF and 0.250 M in LiF. Calculate the pH of the solution after the addition of 0.150 moles of solid LiOH. Assume no volume change upon the addition of base. The Ka for HF is 3.5 * 10-4.

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20 March, 22:14

Question 3) A 1.00 L buffer solution is 0.250 M in HF and 0.250 M in LiF. Calculate the pH of the solution after the addition of 0.150 moles of solid LiOH. Assume no volume change upon the addition of base. The Ka for HF is 3.5 * 10-4.

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Theodore Benson · Answer 1

The pH of the solution after adding 0.150 moles of solid LiF is 3.84

Explanation:

We have the chemical equation,

HF (aq) + NaOH (aq) - >NaF (aq) + H2O

To find how many moles have been used in this

c = n/V=> n = c. V

nHF=0.250 M⋅1.5 L=0.375 moles HF

Simillarly

nF=0.250 M⋅1.5 L=0.375 moles F

nHF=0.375 moles - 0.250 moles=0.125 moles

nF=0.375 moles+0.250 moles=0.625 moles

[HF]=0.125 moles/1.5 L=0.0834 M

[F-]=0.625 moles/1.5 L=0.4167 M

To determine the problem using the Henderson - Hasselbalch equation

pH=pKa+log ([conjugate base/[weak acid])

Find the value of Ka

pKa=-log (Ka)

pH=-log (Ka) + log ([F-]/[HF]

pH = - log (3.5 x 10 ^4) + log (0.4167 M/0.0834 M)

pH=-log (3.5 x 10 ^4) + log (4.996)

pH = - 4.54+0.698

pH = - (-3.84)

pH=3.84

The pH of the solution after adding 0.150 moles of solid LiF is 3.84