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Thread: pH

  1. #1
    Booty Connoisseur DatAtarLyfe's Avatar
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    pH

    Question: What is the pH of a solution containing a mixture of 25.0mL of 0.25mol/L HNO3 and 20.0mL of 0.35mol/L HCl?

    So i found the moles of HCl = 0.007, then found the moles of HNO3 = 0.00625.
    I then subtracted the moles of HNO3 from the moles of HCl = 0.00075 moles in excess of HCl. This means the moles of the solution will be 0.00075 and be acidic.
    I then found the volume of the solution which is 0.025 + 0.02 = 0.045L, which i then used to find the concentration of the solution:
    C = 0.00075/0.045 = 0.16666666...
    I then plugged it into pH = -log[H+] = 1.77815125 = 1.8.

    The problem is that the answer is 0.5. Can someone look over my working and point any flaws in my logic, i'm pretty sure its not a numerical error coz i double checked.

    thanks
    I am rather fond of a sizeable Gluteus Maximus and I am incapable of uttering a falsehood

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    Ancient Orator leehuan's Avatar
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    Re: pH

    Quote Originally Posted by DatAtarLyfe View Post
    Question: What is the pH of a solution containing a mixture of 25.0mL of 0.25mol/L HNO3 and 20.0mL of 0.35mol/L HCl?

    So i found the moles of HCl = 0.007, then found the moles of HNO3 = 0.00625.
    I then subtracted the moles of HNO3 from the moles of HCl = 0.00075 moles in excess of HCl. This means the moles of the solution will be 0.00075 and be acidic.
    I then found the volume of the solution which is 0.025 + 0.02 = 0.045L, which i then used to find the concentration of the solution:
    C = 0.00075/0.045 = 0.16666666...
    I then plugged it into pH = -log[H+] = 1.77815125 = 1.8.

    The problem is that the answer is 0.5. Can someone look over my working and point any flaws in my logic, i'm pretty sure its not a numerical error coz i double checked.

    thanks
    That's an acid and an acid. They don't neutralise! Whoops!

    Resultant volume = 0.045L as you already pointed out

    Resultant moles of acid = 0.007 + 0.00625 = 0.01325mol

    [H3O+] = resultant concentration of acid (both strong acids) = 0.0135/0.045 = 0.3 mol L^-1
    pH = -log_10[0.3] = 0.5228787453...
    DatAtarLyfe likes this.

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    Booty Connoisseur DatAtarLyfe's Avatar
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    Re: pH

    Oh wow. That was pleb.
    Thanks for that
    Nailgun likes this.
    I am rather fond of a sizeable Gluteus Maximus and I am incapable of uttering a falsehood

    Quote Originally Posted by Carrotsticks View Post
    I too enjoy sliding up and down smooth vertical poles and creating lots of tension.

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