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12 November, 14:25

Find the pH of 0.135M NaCN solution. For HCN, Ka=4.9⋅10-10,

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  1. 12 November, 17:21
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    NaCN is considered as salt of weak acid (HCN) and strong base (NaOH):

    we have salt concentration Cs = 0.135 M NaCN and Ka = 4.9 x 10⁻¹⁰

    use this formula to calculate pH:

    pH = 1/2 pKw + 1/2 pKa - 1/2 pCs

    pKw = - log Kw, pKa = - log Ka and pCs = - log Cs

    pKw = - log (1 x 10⁻¹⁴) = 14

    pKa = - log (4.9 x 10⁻¹⁰) = 9.31

    pCs = - log (0.135) = 0.87

    pH = (1/2 x 14) + (1/2 x 9.31) - (1/2 x 0.87) = 11.22

    Note that it is basic solution (pH more than 7) because it consists of strong base and weak acid
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