Chemistry Chapter 1: Water PDF

# Chapter 1: Water ## 1.3 Osmosis ### 36 **Water enriched in salts** **is excreted into the sea** <br> **Inflow of seawater** **Desalination by passing water under pressure** **through filters** **(semipermeable membrane)** <br> <br> **Drinking water** <br> **Dissolved substance*...

# Chapter 1: Water ## 1.3 Osmosis ### 36 **Water enriched in salts** **is excreted into the sea** <br> **Inflow of seawater** **Desalination by passing water under pressure** **through filters** **(semipermeable membrane)** <br> <br> **Drinking water** <br> **Dissolved substance** **Water molecules** **Desalinated water** **FIGURE 1.17 a. Arrangement** **for seawater desalination on a large scale** **(top left).** **b. When seawater enters the** **system under pressure, then pure water** **(blue spheres) passes through the** **semipermeable membrane and ends up** * **in the drinking water tank,** **while salts (red spheres) remain outside** **the system** **(bottom left).** <br> **Biological significance of osmosis** The phenomenon of osmosis plays an important role in many biological phenomena related to cell function. The cell membrane is a semipermeable membrane. That means it allows the passage of water molecules but not protein molecules or other macromolecules* To avoid morphological changes in erythrocytes, as shown in the following figure, injectable solutions (such as physiological saline solution (0.9% w / w NaCl) ) need to have the same osmotic pressure as blood, i.e. about 8 atm. *1. If the cell is immersed in an isotonic solution relative to the intracellular fluid (e.g., saline solution), then the cell maintains its size, since the number of water molecules entering equals the number of water molecules leaving.* *2. If the cell is immersed in a hypertonic solution relative to the intracellular fluid (e.g., thick sugar solution), then the cell shrinks, because the number of water molecules entering is less than the number of water molecules leaving.* *3. If the cell is immersed in a hypotonic solution relative to the intracellular fluid (e.g., pure water), then the cell swells because the number of water molecules entering is greater than the number of water molecules leaving.* *Above. Internal space of a modern desalination unit with reverse osmosis. Below. Each cylinder consists of several million hollow fibers of semipermeable membrane.* ### 37 *The number of water molecules entering is greater than the number of water molecules leaving* *This can lead to cell rupture. For example, if erythrocytes are immersed in pure water, they swell and burst, while hemoglobin (red pigment) contained in them is dissolved in water. This is called hemolysis of red blood cells.* *1.* *2.* *3.* **FIGURE 1.18 Erythrocytes in solution a. isotonic b. hypertonic and c. hypotonic with respect to intracellular fluid.** ### Example 1.7 A horizontal cylindrical container is 24 cm long and is divided in the middle by a movable semipermeable membrane. One part is filled with an aqueous solution containing 0.1 mol of substance A and the other with an aqueous solution containing 0.2 mol of substance A. Which way and how much will the membrane move? Initial state | |12 cm |12 cm | | |---------|-------|-------|-----------| | | 0.1 mol A | 0.2 mol A | a cm² | | | | | | | | | | | Final state | | | | | |---------|----------|---------|-------| | | (12-x) cm |(12+x) cm| a cm² | | | | | | | | | | | **SOLUTION** According to the osmosis phenomenon, water molecules will pass through the semipermeable membrane from the solution of lower concentration to the solution of higher concentration. The phenomenon will stop when the concentrations of the solutions on both sides of the membrane are equalized. We say it will stop, because a dynamic equilibrium will be established, meaning that the same number of solvent molecules pass through the semipermeable membrane in both directions at the same time. For that to happen in our case, the membrane will move to the left to make room for the water, as shown in the figure. Suppose it moves by x cm. The amount of solute A will remain the same in both solutions and in the final state, we have: C₁=C₂ or 0,1 mol A = 0,2 mol A (12-x)a cm³ (12+x)a cm³ or x = 4, which means the membrane will move 4 cm to the left. ### 38 *Concentration in the dissolved substance. The phenomenon, as we mentioned, will stop when the concentrations of the solutions on both sides of the membrane are equalized*. *When we say it will stop, we mean that a dynamic equilibrium will be established, meaning that the same number of solvent molecules pass through the semipermeable membrane in both directions at the same time.* *For that to happen in our case, the membrane, as shown in the figure (b), will move to the left to make room for the water.* *Suppose it moves by x cm. The amount of solute A will remain the same in both solutions and in the final state, we have:* *C₁=C₂ or* *0,1 molA = 0,2 molA* *(12-x)a cm³ (12+x)a cm³* *or x = 4, which means the membrane will move 4 cm to the left.* ### Application - A horizontal cylindrical container is 40 cm long and is divided in the middle by a movable semipermeable membrane. - One part is filled with an aqueous solution containing 0.2 mol of substance A and the other with an aqueous solution containing 0.5 mol of substance B. - Which way and how much will the membrane move and why? **8.57 cm**