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Question

100 ml of 0.2 molar acetic acid with 50 ml of 0.1 M and magnesium Hydroxide calculate the pH of the final solution

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Solution

Start by determining the concentrations of the two species in the buffer. For acetic acid, you have

C=n/V⇒nacetic
=C⋅V
=0.2M⋅100⋅10^−3L
=20⋅10^−3moles

This means that the concentration of acetic acid in the buffer will be

Cacetic=nacetic/Vbuffer
=20⋅10^−3moles/(100+100)⋅10^- 3L
=0.1 M

For sodium acetate, the number of moles is

nacetate=C⋅V
=0.15xMx100⋅10^−3L
=15⋅10−3moles

As a result ,the concentration of MgOH in the buffer will be

CMgOH=nMgOH/Vbuffer
=10⋅10^−3moles/(100+50)x10^-3
= 0.6667

Since you're dealing with a buffer, use the Henderson-Hasselbalch equation to determine the pH of the solution

pHsolution=pKa+log([MgOH][CH3COOH])

pHsolution=4.75+log(0.0667M ÷0.1M)
=4.75−0.175=4.575



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