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
2.50 microgram= 0.000125 grams
((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 ?