Solubility Information Pt.II There is another term that is very rarely used to express solubility and that term is MOLALITY. Molality (m) is defined as the number of moles of solute per kilogram of solvent. –m = moles of solute / kilograms of solvent Ex: You are asked to determine the MOLALITY of a NaCl solution made up of 20g of NaCl dissolved in 100g of water.

First, you have to convert the 100g to kilograms (kg)  0.10kg. Then you have to determine the number of moles of NaCl – (58.44 g/mole)  20g ÷ 58.44g/mole = 0.34moles Molality (m) = 0.34moles ÷ 0.10kg = 3.4 m Please note molality is expressed as m

Parts per million & parts per billion Although we can use percentages and molarity ( or even molality) to express concentrations in dilute solutions, they don’t work very well when we look at very dilute solutions. Typically we find very dilute solutions in nature and so 2 new units of concentration were developed: parts per million (ppm) and parts per billion (ppb)

The units ppm and ppb are used most frequently when describing the level of contaminants found in the environment. Ppm is defined as the grams of solute per one million grams of solvent. It is most commonly expressed as milligrams of solute per kilogram of solvent. For example, the level of lead in drinking water used to be 0.05ppm – or 0.05 milligrams of lead per litre. This level has been revised to 0ppm. Ppb (parts per billion) is expressed as micrograms of solute per kilogram of solvent. Neurotoxins are usually expressed in these terms – Fugu toxin (from puffer fish) has a lethal dose of 8 micrograms per kilogram of body mass.

What colligative properties of solutions are and what they aren’t. Some properties of solutions are dependent on the properties of the solute. For example, salt solutions taste salty, acidic solutions taste sour, and basic solutions taste bitter. Some solutions conduct electrical currents and are called electrolytes. Ex:Saline solutions are electrolytes but sugar solutions are not. Although these are properties of solutions, they are not colligative properties.

Properties that depend on the relative number of solute particles are colligative properties. The effect you record about a solution is dependent on the number of solute particles present. Examples are: The lowering of vapor-pressure – A liquid stored in a closed container will eventually evaporate (to a certain extent) and there will be a gaseous layer above the remaining liquid. This gaseous layer will press down on the liquid – this is referred to as the vapor pressure. If you make this liquid the solvent in a solution, then the addition of a solute will cause a lowering of the vapor pressure above the solution because molecules of the solvent can’t evaporate as easily.

Boiling point elevation Every liquid has a given boiling point – water boils at 100°C, or ethyl alcohol has a boiling point of 78.37°C. If you use a specific liquid, like water or alcohol, as a solvent, the addition of a solute will elevate the boiling point. An example would be anti-freeze – the addition of ethylene glycol to water will raise the boiling point of water so it can be used very nicely in car radiators to keep engines cool.

Freezing point depression All liquids also have freezing points – for water it is 0°C, and for alcohol it is -114°C. The addition of a solute to a pure liquid will always depress the freezing point. As an example, rock salt is added to the ice/water mixture to form a solution with a lowering freezing point than water – when making ice cream.