Converting mm Hg to mm H2O

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This topic contains 1 reply, has 0 voices, and was last updated by  Ethon 12th May 2018 at 8:50 am.

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    Ethon
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    So here’s my problem :

    What is the osmotic pressure, noted in mm of H2O, of a solution with 125 micrograms of vitamin B12 (C63H88CoN14O14P) in 2.50 mL of H2O at 25 degree celsius.

    The “hint” that comes with the problem says : Weak osmotic pressures are often measured and given in mm of H2O. Use here the density of water (1,00 g/mL) and of Hg (13,6 g/mL). In this case, the constant R has a value of 62,36 mm Hg * L / mol * K

    Here :

    2.50 microgram= 0.000125 grams

    pi=cRT

    ((0.000125 gram * 1 mol / 1355.3652 g) / (2.50 mL / 1000)) * 62.36 mm Hg * L / mol * K * 298 . 15 K = 0.685890194 mm Hg

    So, I blocked here and I saw that the solution was :

    0.685890194 mm Hg * 13.6 g/ml / 1.00 g/ml =9.32 mm H2O

    So, here’s two questions :

    Why did I have to use R =62.36 and not the usual R=8.31

    Also, what’s happening in the conversion exactly ? How was I supposed to figure out what they did to convert from mm Hg to mm H2O ?

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