Osmotic pressure is the pressure applied by a solution to prevent the inward flow of water across a semipermeable membrane.

The osmotic pressure is expressed by the formula:
$\Pi =\mathrm{iMRT}$
where:
Π is the osmotic pressure in atm
i = van 't Hoff factor of the solute.
M = molar concentration in mol/L
R = universal gas constant = 0.082057 L atm mol^-1 K^-1
T is the temperature of the gas (measured in Kelvin).

The universal, gas constant (R) is a numerical value made up of the combination of atandard pressure (1 atmosphere or 760 torr), stanard temperature (273 K), and the fact that 1 mole of a gas occupies a volume of 22.4 liters at STP.

Example 1:
10 grams of glucose is dissolved in 200 ml of water. What is the osmotic pressure at 30 °C?
i = van 't Hoff factor for glucose i = 1
molar mass of glucose (C⁶H¹²O⁶) = 6(12) + 12(1) + 6(16) = 180 g/mol
n = Moles for glucose = 10 g x 1 mol/180 g = 0.0555 mol
M = molar concentration in mol/L = mol/Volume = 0.0555/(200 ml x 1 L/1000 ml) = 0.0555/0.2L = 0.2775 mol/L
R = universal gas constant = 0.082057 L atm mol^-1 K^-1
T is the temperature of the gas (measured in Kelvin) = 30 + 273 = 303 K
$\Pi =\mathrm{iMRT}$

$\mathrm{Osmotic\; pressure\; \Pi }=1x\mathrm{0.2775\; mol/L}x{\mathrm{0.082057\; L\; atm\; mol}}^{-1}{K}^{-1}x\mathrm{303\; K}=6.899\mathrm{atm}$

Osmotic Pressure is based upon absolute temperature. The absolute temperature is calculated by adding 273 to the temperature in Celsius scale. 10°C is equiavalent to 283 K.

$T_{}=10\mathrm{°C}+273=283K$