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20 October, 12:49

30 ml of a 0.5 N lithium hydroxide standard are used to titrate 25 ml of a phosphoric

acid sample. What is the molarity, normality and pH of the acid?

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Answers (1)
  1. 20 October, 15:42
    0
    0.200M H₃PO₄

    0.600N H₃PO₄

    pH = 1.46

    Explanation:

    The neutralization reaction of phosphoric acid (H₃PO₄) with LiOH is:

    3 LiOH + H₃PO₄ → Li₃PO₄ + 3H₂O

    Where 3 moles of LiOH reacts per mole of H₃PO₄

    Moles of LiOH used in the standarization are:

    0.030L * (0.5mol / L) = 0.0150 moles of LiOH

    Moles of acid are:

    0.0150 moles of LiOH * (1 mole H₃PO₄ / 3 moles LiOH) = 0.005 moles H₃PO₄

    As volume of acid was 25mL, molarity is:

    0.005mol H₃PO₄ / 0.025L = 0.200M H₃PO₄

    Normality is:

    0.200M * (3N H⁺ / 1M H₃PO₄) = 0.600N H₃PO₄

    H₃PO₄ dissolves in water thus:

    H₃PO₄ ⇄ H₂PO₄⁻ + H⁺

    Ka = 7.1x10⁻³ = [H₂PO₄⁻] [H⁺] / [H₃PO₄]

    Where molar concentrations in equilibrium will be:

    [H₂PO₄⁻] = X

    [H⁺] = X

    [H₃PO₄] = 0.200M - X

    Replacing:

    7.1x10⁻³ = [X] [X] / [0.200 - X]

    1.42x10⁻³ - 7.1x10⁻³X = X²

    0 = X² + 7.1x10⁻³X - 1.42x10⁻³

    Solving for X:

    X = - 0.04M →False answer, there is no negative concentrations.

    X = 0.0343M

    Thus, [H⁺] = 0.0343M

    As pH = - log [H⁺],

    pH = 1.46
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