Osmotic Pressure: Definition & Formula
What Is Osmotic Pressure?
Osmosis is the movement of water from an area of low concentration of solute to an area of higher concentration of solute. A solute is atoms, ions, or molecules dissolved in a liquid. The rate of osmosis is determined by the total number of particles dissolved in the solution. The more particles dissolved, the faster the rate of osmosis.
If a membrane is present, water will flow to the area with the highest concentration of solute. Osmotic pressure is the pressure created by water moving across a membrane due to osmosis. The more water moving across the membrane, the higher the osmotic pressure.
How Is Osmotic Pressure Calculated?
Osmotic pressure can be determined using the equation shown here.
Osmotic pressure is affected by concentration and temperature.
Osmotic pressure formula
Concentration of solute and temperature each affect the amount of pressure created by the movement of water across a membrane. Higher concentrations and higher temperatures increase osmotic pressure.
Osmosis is also impacted by how the solute behaves in water, which is where Van’t Hoff’s factor comes in. Basically, the Van’t Hoff’s factor of a solute is determined by whether or not a solute stays together or breaks apart in water. Some solutes break apart and form ions, or charged atoms, in water. As shown here, table salt, NaCl, will form sodium (Na+) and Chlorine (Cl-) ions in water. The Van’t Hoff’s factor of NaCl is two because it breaks into two ions.
When placed in water, some molecules such as sucrose stay together and do not form ions. Since sucrose does not break into ions, it has a Van’t Hoff’s factor of 1.
Example Problems
What is the osmotic pressure for a 0.5 M sucrose solution at 300K (27°C) when sucrose has a Van’t Hoff factor of 1?
All you need to do is plug what you know into the equation to solve for osmotic pressure, which is this simple (pi).
i would be 1 since sucrose does not form ions in water
M would be 0.5 mol/L, or the concentration of sucrose you are using.
R is the ideal gas constant 0.08206 L atm/mol K
T is 300 K
Since all of the other units cancel each other out, osmotic pressure is measured in atms.
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