Notes - Solutions Mr. Forte Atascadero High School Chemistry

Definitions Solute: Substance that is dissolved. Solvent: Substance doing the dissolving. Solution: A homogeneous mixture of a solute and solvent. Saturated: Solution holding maximum amount of solute at current conditions. Concentration: Measure of the amount of a solute dispersed in a given amount of solvent.

Factors Affecting Solubility Polarity: Like dissolves like. Temperature: May increase or decrease amount of solute that will dissolve. Concentration: Once a solution is saturated, no more solute dissolves. A solid may precipitate or a gas may bubble out. Pressure: The solubility of a gas increases with an increase of gas pressure above solution.

Increasing Rate of Dissolving Increase Surface Area – Crushing a solid helps it dissolve faster. Stirring – Allows particles to leave surface of solid faster. Increase Temperature - Increased kinetic energy allow particles to leave solid surface faster.

Expressing Concentration Molarity Molality Percent by Mass Parts per Million (or Billion, etc)

Molarity (M) Expresses concentration by indicating the moles of solute per liter of solution. (moles of solute/liters of solution) Most common way to express solution concentrations in chemistry. A 3.0 molar sodium chloride solution indicates that there are three moles of sodium chloride per liter of solution.

Example What is the molarity of a solution that has 3.0 moles of solute in 2.0 liters of solution? Molarity = (3.0 mol solute/2.0 L solution) = 1.5 M

Molality (m) Expresses concentration by indicating the moles of solute per kilogram of solvent. (molality = moles of solute/kg of solvent) Unlike molarity, molality is not temperature dependent.

Example What is the molality of a solution containing 1.5 moles of sugar in 300 grams of water? Molality = 1.5 mol solute/0.300 kg solvent = 5.0 m

Percent by Mass Expresses concentration by indicating the solute’s mass as a percentage of the mass of the solution. A 5% salt solution means that of the total mass of the sample, 5% of the mass is from salt. Most common way to express concentrations of household chemicals. To calculate, divide mass of solute by mass of solution, then multiply by 100.

Example What is the percent by mass of ammonia in a solution that contains 12 grams of ammonia dissolved in 150 grams of water? (Note: mass of solution = solute + solvent) Percent by Mass (aka, parts per hundred) = (12 g solute/162 g solution) x 100 = 7.4%

Parts per Million (ppm) Indicates mass of solute per one million mass units of solution. For example, 3 ppm indicates that there are 3 grams of solute in every one million grams of solution. To calculate, divide mass of solute by mass of solution, then multiply by one million.

Example What is the parts per million of lead in drinking water that contains 0.03 grams of lead in a 3800 gram sample? Note: Parts per billion (ppb) or Parts per trillion (ppt) work the same way! Parts per million: = (0.03 g Pb /3800 g solution) x = 7.9 ppm

Solution Process Forming a solution requires three steps: Separating the solute (requires energy) Separating the solvent (requires energy) Mixing of solute-solvent particles (releases energy) The amount of energy involved in each step determines the Heat of Solution.

Heat of Solution When a solute dissolves in a solvent, there is always an absorption or release of energy. The Heat of Solution can be calculated by determine the enthalpy change (  H) per gram or mole of solute dissolved. The amount of heat released or absorbed can be determined using Q = m x s x  T. Heat of Solution = Q/grams or moles of solute.

Diluting Solutions M 1 V 1 = M 2 V 2 –Where ones represent before and the twos represent after. This formula is very versatile and allows you to determine new concentrations or volumes given changing conditions. See board for example.

Preparing Solutions Determine amount and concentration needed. From scratch: Weigh out solute/solvent Completely dissolve solute in solvent Added solvent (water) to desired volume

Electrolyte Solutions Solutions that conduct an electrical current are called electrolytic solutions. Since they contain electrolytes. (charged particles, like ions.) A conductivity meter allows for a relative determination of the number of electrolytes present. Solution that do not conduct electricity contain nonelectrolytes. (Most molecular solutes, like sugars, fall into this category.)

Colligative Properties Properties of a solution that depend only on the number of solute particles present, not their identity, are called colligative properties. Examples include freezing-point depression, boiling point elevation, and vapor pressure. –Solutions always boil at a higher temperature and freeze at a lower temperature relative to the pure solvent.