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The pH of a solution obtained by mixing 100cm\(^3\) of a 0.1M HCl solution with...

Chemistry
JAMB 1998

The pH of a solution obtained by mixing 100cm\(^3\) of a 0.1M HCl solution with 100cm\(^3\) of a 0.2M solution of NaOH is?

  • A. 1.3
  • B. 7.0
  • C. 9.7
  • D. 12.7
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Correct Answer: Option D
Explanation

Calculate Moles of Reactants:

Moles of HCl = Molarity x Volume in dm\(^3\) = 0.1 M x  0.1 dm\(^3\) = 0.01 moles

Moles of NaOH = Molarity x Volume in dm\(^3\) = 0.2 M x 0.1dm\(^3\) = 0.02 moles

Determine the Excess Reagent:

The neutralization reaction is a 1:1 ratio: HCl + NaOH → NaCl + H\(_2\)O

Excess NaOH = 0.02 - 0.01 = 0.01 mole

Calculate the Hydroxide Ion Concentration[OH\(^-\)]

Total Volume of the mixture = 100 cm\(^3\) + 100cm\(^3\) = 200 cm\(^3\) = 0.2dm\(^3\)

[OH\(^-\)] = \(\frac{0.01}{0.2}\) = 0.05M

Calculate pOH and pH

pOH = - Log[OH\(^-\)] = - Log[0.05]

        = 1.3

But   pOH + pH  = 14

          1.3 + pH = 14

            pH = 14 - 1.3 

                  = 12.7

 


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