Solubility and Molarity

Solubility:

Saturated Solution:

  • Maximum amount of solute has been dissolved in the solvent at that temperature

  • The solution has reached a state of equilibrium

  • Rate of dissolution = rate of crystallization (dissolving = crystalizing)

  • So, there is a constant concentration of solute dissolved in a saturated solution because the rate of dissolving/undissolving is the same

  • The maximum amount of solute that is dissolved in a saturated solution is different for every substance

  • One can measure the solubility of a substance by max grams of solute dissolved divided by 100g of water

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Unsaturated Solution:

  • Less than the maximum amount of solute possible

  • More solute can be dissolved in the solution (when it is below the curve)

  • The line is the max at which it can dissolve

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Supersaturated Solution:

  • More solute than is normally possible in a saturated solution

  • Unstable solution with more solute dissolved than a saturated solution at that temperature (above the curve but all dissolved)

  • To disturb, add a little extra solid, bend disk (release a tiny crystal), shake the solution

  • To make, heat up the solute-solvent mixture until all the solute dissolves then cool slowly (solid should stay dissolved unless disturbed)

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  • For ionics/solids, as temp increases, solubility increases because more entropy

  • For covalents/gas, as temp increases, solubility decreases because freeing up gas/less entropy

  • For gases, as pressure increases, solubility increases (Henry’s law)

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Molarity:

  • A measure of the concentration of a solution

  • Moles of solute per liter of solution

  • More moles dissolved per liter, more concentrated the solution

  • M = moles/L

  • Temperature dependent, when temp increases, volume increases

  • One uses volumetric flasks to make solutions with precise concentrations

  • Use M1V1 = M2V2 when we do not have a solid form of the substance we want to dissolve so we need a very concentrated solution from a chemical company to dilute it and get the molarity we want

  • Put it in the volumetric flask, fill to mark, shake to dissolve \n

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Why Ice Melts:

  • Freezing point depression

  • Salt lowers the freezing point of water

  • Normally water is 0 degrees celsius

  • Freezing point goes down to below outside temperature

  • Ice melts

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Why is pure water easier to freeze than other water?

  • Ice has a hexagonal structure

  • If particles get in the way/disrupt the formation then it is harder to freeze

  • So lower temp is needed

  • Freezing point of water is lower when the solution has a higher concentration of the same solute

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Freezing point depression:

  • The amount of a freezing point that goes down when a solute is added is not dependent on the identity of the solute but dependent on the concentration of solute particles

  • Higher molarity = lower freezing point

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Boiling point elevation:

  • The amount the boiling point increases is not dependent on the identity of the solvent but is instead only dependent on the amount of solute
  • Higher molarity = highest boiling point
  • When more solute is added the boiling point increases

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Why do solutions have a higher boiling point than pure water?

  • Solutions have more solute in them which makes it harder for water molecules to enter the gas phase.

  • The solution has a slower rate of evaporation and low vapor pressure (a measure of the tendency of a material to change into the gaseous or vapour state)

  • WHen solute particles are added some reach the upper layer of hte solution so the vapor pressure lowers due to the decrease in solvent molecules at the surface

  • Harder for water molecules to evaporate = more energy needed = higher bp

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Absorbance:

  • Higher concentration means higher absorbance because the proportion of light that gets absorbed is affected by the number of molecules that it interacts with.

  • There are more molecules that interact with light

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Scuba Diving:

  • When a diver goes underwater, more nitrogen gas dissolves in his blood because Henry’s Law explains that during a dive any gases entering the blood are absorbed in the diver’s blood. There is an increase in solubility because higher pressure

  • When the diver comes up pressure decreases and volume increases. Bubbles form in the bloodstream because the gas is trying to escape.

  • Increased solubility of a gas at a higher pressure may have narcotic effects because there is too much nitrogen in the bloodstream.

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Van’t Hof Factor:

  • Number of particles of th solute that breaks up in solutions

  • Count number of particles (check for big ones)

  • Ionic compounds are one

  • Multiply by molarity

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