| Osmosis |
If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis.
Osmosis is of great importance in biological processes where the solvent is water. The transport of water and other molecules across biological membranes is essential to many processes in living organisms. The energy which drives the process is usually discussed in terms of osmotic pressure.
Read the full article. |
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If you concentrate just on the molecules that can cross the membrane (the small black dots) you will notice that in the top image they are more concentrated on the left side, than the right. Hence, osmosis wants to make the two sides equal with respect to JUST the black dots, so they begin to move from the left to the right side through the membrane, and we end up with the bottom situation. The volume of the right side has increased. |
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| Molarity |
A way of expressing concentration is called molarity. Molarity is the number of moles of solute dissolved in one liter of solution. The units, therefore are moles per liter, specifically it's moles of solute per liter of solution.
molarity = |
moles of solute |
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liter of solution |
Rather than writing out moles per liter, these units are abbreviated as M or M. We use a capital M with a line under it or a capital M written in italics. So when you see M or M it stands for molarity, and it means moles per liter (not just moles).
You must be very careful to distinguish between moles and molarity. "Moles" measures the amount or quantity of material you have; "molarity" measures the concentration of that material. So when you're given a problem or some information that says the concentration of the solution is 0.1 M that means that it has 0.1 mole for every liter of solution; it does not mean that it is 0.1 moles. Please be sure to make that distinction.
Example #1 - Suppose we had 1.00 mole of sucrose (it's about 342.3 grams) and proceeded to mix it into some water. It would dissolve and make sugar water. We keep adding water, dissolving and stirring until all the solid was gone. We then made sure that when everything was well-mixed, there was exactly 1.00 liter of solution.
What would be the molarity of this solution?
The answer is 1.00 mol/L. Notice that both the units of mol and L remain. Neither cancels.
A replacement for mol/L is often used. It is a capital M. So if you write 1.00 M for the answer, then that is correct.
Some textbooks make the M using italics and some put in a dash, like this: 1.00-M. When you handwrite it; a good, old block capital M is just fine.
And never forget this: replace the M with mol/L when you do calculations. The M is just shorthand for mol/L. |
Molar (M) solutions are based on the number of moles of chemical in 1 liter of solution. A mole consists of 6.02x100000000000000000000000 (23) molecules or atoms. Molecular weight (MW) is the weight of one mole of a chemical. Determine MW using a periodic table by adding the atomic mass of each atom in the chemical formula. Example: For the MW of CaCl2, add the atomic mass of Ca (40.01) to that of two Cl (2 x 35.45) to get 110.91 g/mole. Therefore, a 1M solution of CaCl2 consists of 110.91 g of CaCl2 dissolved in enough water to make one liter of solution.
Once the molecular weight of a chemical is known, the weight of chemical to dissolve in water for a molar solution less than 1M is calculated by the formula:
grams of chemical = (molarity of solution in mole/liter) x (MW of chemical in g/mole) x (ml of solution) ÷ 1000 ml/liter
For example, to make 100 ml of 0.1 M CaCl2 solution, use the previous formula to find out how much CaCl2 you need:
grams of CaCl2 = (0.1) x (110.91) x (100) ÷ (1000) = 1.11 g
Now you can make your solution: dissolve 1.11 g of CaCl2 in sufficient water to make 100 ml of solution. The amount of water needed will be slightly less than 100 ml. |
| Tonicity |
1. Normal firmness or functional readiness in body tissues or organs.
2. The sustained partial contraction of resting or relaxed muscles.
View this animation to help you understand. |
Tonicity is the ability of a solution to cause water movement. It is in reference to hypertonic, hypotonic and isotonic cellular states.
The elastic tension of living muscles, arteries, etc. that facilitate response to stimuli. |
