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$\ce{CuSO4}$ is acidic salt because it is made up of $\ce{Cu(OH)2}$ (weak base) and $\ce{H2SO4}$ (strong acid). But why $\ce{[CuSO4.4H2O]H2O}$ is amphoteric?

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    $\begingroup$ How is the pentahydrate amphoteric? $\endgroup$ Jul 13, 2019 at 14:16
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    $\begingroup$ Before you ask "why", ask "if". $\endgroup$ Jul 13, 2019 at 15:29
  • $\begingroup$ Hint: you may substantiate your question by addition of experimental evidence. Did you dissolve the same quantity (referring to moles) of either $\ce{CuSO4}$ or its pentahydrate in the same quantity of water and determine the pH value of these solution with pH electrodes? Maybe you have better equipment at disposition, but let us assume the typical error of such a determination is less or equal to 0.05 pH units, how many pH units the two solutions differ from each other? $\endgroup$
    – Buttonwood
    Jul 13, 2019 at 16:43

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The formula weight of $\ce{CuSO4}$ is 159.609 g/mol, that of $\ce{CuSO4 . 5 H2O}$ is 249.685 g/mol and the molecular weight of $\ce{H2O}$ is 18.015 g/mol. So suppose you have two 500 mL beakers and you put 200.0 mL of water in each one. To the first beaker, add 15.9609 g of $\ce{CuSO4}$ and 9.008 g of water. Stir until all the copper sulfate has dissolved. The resulting blue solution has some pH, whatever it is. Now to the second beaker, add 24.9685 g of $\ce{CuSO4 . 5 H2O}$. Stir until all the copper sulfate pentahydrate is dissolved. The two solutions have the same intensity of blue color, because their $\ce{Cu^2+}$ concentrations are the same, and their pH values are the same. So, like @Oscar Lanzi says, how is the pentahydrate amphoteric?

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    $\begingroup$ Exactly. The original poster seems to have a major misconception regarding the concept of pH in water. $\endgroup$
    – AChem
    Jul 13, 2019 at 17:11

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