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Solutions Class #4

Solutions Class #4. Parts Per Million and then, The colligative properties of water.

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Solutions Class #4

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  1. Solutions Class #4 Parts Per Million and then, The colligative properties of water

  2. You have a very weak salt water solution that contains just 152.3 grams NaCl dissolved into 312,400 liters of water. What is the molarity of this solution? (the answer is stupidly small to make a point, do your best with the blue calculators)

  3. You have a very weak salt water solution that contains just 152.3 grams NaCl dissolved into 312,400 liters of water. What is the molarity of this solution? (the answer is stupidly small to make a point, do your best with the blue calculators) moles soluteliters solution 2.626 moles NaCl312,400 Liters M = M = 0.00000841 Molar NaCl(AQ) That’s 841 hundred millionths Molar (got that?) Sort of a stupid number.

  4. A better way to do concentration of a really weak solution is parts per million. The formula is on the back of your reference table, look for it now, copy it into your notes, and redo this same problem, but instead of molarity, what is the concentration of this solution in PPM?

  5. You have a very weak salt water solution that contains just 152.3 grams NaCl dissolved into 31,240 liters of water. What is the molarity of this solution? grams solutegrams solution PPM = x 1,000,000 =

  6. You have a very weak salt water solution that contains just 152.3 grams NaCl dissolved into 312,40 liters of water. What is the molarity of this solution? grams solutegrams solution PPM = x 1,000,000 = 152.3 g NaCl31,240,000 g solution PPM = x 1,000,000 = PPM = 4.875 parts per million NaCl in solution(4 SF)

  7. One tiny CoBr2 crystal has a mass of 0.00600 grams. If you dissolve it into water to form a one liter solution of CoBr2(AQ), what is the molarity of this, what is the solution’s concentration in PPM? (2 formulas first)

  8. One tiny CoBr2 crystal has a mass of 0.00600 grams. If you dissolve it into water to form a one liter solution of CoBr2(AQ), what is the molarity of this, what is the solution’s concentration in PPM? (2 formulas first) moles CoBr2liters solution grams CoBr2grams solution M = PPM = x 1,000,000 0.00600 g CoBr21000 g solution 0.0000274 g CoBr21.00 Liters PPM = x 1,000,000 M = M = 0.0000274 M or 274 ten millionths molar! (3 SF) PPM = 6.00 PPM (3 SF) This is a more “normal” number for the normal brains we’re used to.

  9. Molarity and PPM are equivalent measures, with very different units. My height on a good day is 5 feet 8 inches (or 68.0 inches). That measure in pico meters (which is usually used to measure the radius of atoms, is equal to 2.68 x 1011 pm, or = 268,000,000,000 pm tall. Either way, I am still (rather) tall. But my height in picometers is silly. Use Parts Per Million when the concentrations are crazy low, or Molarity when concentrations are more “normal” ranged. Sometimes you can’t tell until you have done the math.

  10. There are several properties of water that are called the colligative properties. They are the properties that are affected by dissolving solutes into the water. (other solutions will have colligative properties but not in our intro class). These include: Boiling Point Freezing Point Vapor Pressure (how well does the liquid evaporate)

  11. When a bunch of water molecules are close (and not frozen) they are attracted to each other by hydrogen bonding. It magnetically attracts the molecules together. To freeze, boil, or evaporate, the water must “deal with” these bonds. If it’s water, it’s bound to be hydrogen bonding!

  12. Boiling point elevation: To boil water that is pure, the molecules must get a certain amount of energy in joules to vibrate fast enough to overcome both the internal attraction to each other (break the hydrogen bonds) and the air pressure holding the water “down”. The energy required to do this happens at 373 Kelvin at standard pressure. If there is solute dissolved in your water, to make it boil it will require that same amount of energy PLUS the energy to break free of the ions (or polar molecules) that would also be attracted to the water. When solute is dissolved in water there is a boiling point elevation.

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  14. Freezing point depression: To freeze water that is pure, the molecules must lose a certain amount of energy in joules to slow their vibrations enough to allow the hydrogen bonds to become “stronger” than the kinetic energy they have to make them move like Jagger. When that happens, the water molecules form into 6 molecule rings and the molecules lock together (ice). The energy required to do this happens at 273 Kelvin at standard pressure. If there is solute dissolved in your water, to make it freeze it will require that same amount of energy PLUS the removal of even more energy to allow the water molecules to freeze solid. This solute gets in the way of the rings of ice forming, requiring even lower kinetic energy to freeze (colder temperatures). When solute is dissolved in water there is a freezing point depression.

  15. Vapor Pressure is depressed with solute in the water. Water does not evaporate so well (because of those hydrogen bonds). they make the water molecules sticky, and hard to shake free to evaporate. Vapor Pressure is a way to measure how well or poorly a liquid can evaporate. To evaporate, the individual molecules, at the surface, must get enough kinetic energy by colliding to bounce through the surface and into the air. Then they are free. They must over come both the hydrogen bonding and the air pressure holding the molecules down in the water. When solute is dissolved in water it has to be ionic or polar, either way the water must overcome the same stuff it always has (hydrogen bonding and the air pressure) PLUS the attraction to all these ions or polar molecules of solute. The more solute in water, the Vapor Pressure gets depressed. (evaporates more slowly).

  16. You will need to copy these numbers into your reference table, and notes now. For water, The boiling point elevation is 0.50 K/ mole of particles dissolved in 1 liter of solution. The freezing point depression is 1.86 K/mole of particles dissolved in 1 liters solution. In our class, we won’t do math for the vapor pressure, but know… the more solute, the slower that water will evaporate.

  17. You have exactly 1.00 liters of 2.00 NaCl solution. At what temperature will this solution boil and freeze? Hint, first figure out how NaCl will ionize. Each mole of NaCl does not form a mole of particles, it forms 2 moles of particles: a mole of Na+1 cations, AND a mole of Cl-1 anions as well.

  18. You have exactly 1.00 liters of 2.00 NaCl solution. At what temperature will this solution boil and freeze? So, you have 2.00 Moles of NaCl so you have 2 moles of sodium cations and 2 moles of chloride anions. That’s 4 moles of particles. These 4 moles of particles are dissolved into 1.0 liters of solution. Each mole will elevate the Boiling point by 0.50 K so the BP changes to 373 K + (4 x 0.50 K) = 375 Kelvin (3 SF) Each mole of particles will depress the freezing point by 1.86 K, so the FP changes to 273 – (4 x 1.86 K) = 273 k – 7.44 K = 266 K(with 3 SF)

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