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Lecture 1sf

# Lecture 1sf - Solution Homogeneous Mixture Solute That...

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Solution: Homogeneous Mixture Solute: That which dissolves Solvent: Dissolving medium (usually water) The solute can be solid, liquid, or gas. The solvent can also be solid, liquid, or gas. However, solvent is usually a liquid. Solute/solvent Examples gas/gas Air, natural gas gas/liquid CO 2 /water (soda) HCl/water (hydrochloric acid) NH 3 /water (aqueous ammonia) liquid/liquid Ethanol/water (alcoholic beverages) solid/liquid NaCl/water (salt water) naphthalene/benzene solid/solid Ag/Au (14 karat gold) Zn/Cu Brass Pb/Sn Solder Air is not usually described as a solution, but it does fit the definition of a homogeneous mixture (assuming the air is in a reasonably small space, since the air spread over a large distance is not homogeneous). In a liquid/liquid solution, it is unclear as to what dissolves in what (which is the solute, which is the solvent). The two liquids are said to be mutually soluble, or miscible. An ethanol-water mixture is miscible in all proportions. Any amount of ethanol can mix with any amount of water to form a homogeneous mixture. On the other hand, when a solid dissolves in a liquid, there is a limit to its solubility. When maximum solubility has been reached, the solution is said to be saturated.

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Solution Concentration Solutions are mixtures with variable composition. We describe the composition of the solution in terms of concentration. In general: Concentration = where “solution” = solute + solvent Some common units of concentration are: Percent by mass = x 100 This is equivalent to “parts per hundred.” where “parts” refer to mass. For really small concentrations (trace amounts), we sometimes use ppm (parts per million) or even ppb (parts per billion). ppm = x 10 6 ppb = x 10 9 In aqueous solutions, ppm is equivalent to L mg . Each liter of water has a mass of 1 kg—so L mg is equivalent to kg mg or 3 3 10 10 - or 6 10 1 or ppm. Other units used by chemists are molarity, molality and mole fraction. Molarity = Molality = Mole fraction = Add 5.00 g NaCl to 100 g of water. Calculate the percent by mass, molality and mole fraction.
Percent by mass = x 100 = 4.76% Molality = = 0.855 m Mole fraction = = 0.0152 In the above expression represents mol NaCl and represents mol water. What additional information must be given in order to calculate the molarity of this solution? We would need to know the solution’s density. Since molarity is , we require information relating volume to amount. The density relates volume to mass, and molar mass can then be used to relate mass to moles. This is illustrated in the next example. Concentrated hydrochloric acid is 37.0% HCl by mass. Its density is 1.19. Calculate the molarity, molality, and mole fraction of HCl. The density of 1.19 is equivalent to 1190 . Each liter has a mass of 1190 g, and 37.0% of that mass is HCl. Thus: Molarity = 0.37 (1190 ) x = 12.1 M An alternate, equivalent way of solving is to start with thinking of 37% as .

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