[moop1]

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 ?

[billnotgatez]

Check this WIKI

http://en.wikipedia.org/wiki/Gas_constant

also these links may be of interest

http://www.efunda.com/formulae/fluids/manometer.cfm
http://online.unitconverterpro.com/conversion-tables/convert-alpha/pressure.html

[Borek]

Numerical value of every physical quantity depends on the units used to express it.

Actually it doesn't matter which R version do you use, you can convert the pressure later. In this particular case, you are given information that makes conversion easy for pressures expressed in mmHg and mmH₂O, so the choice of R expressed using mmHg looks the most reasonable.

Do you know how to express pressure exerted by a column of a liquid of a height h and density ρ? (This is one of these things that make basic physics prerequisite to the chemistry).