Concentrations and Solubility

Concentrations• Concentrations: Measures the amount of solute per

amount of solvent

• Concentrated solutions has “a lot” of solute dissolved• Dilute solutions has “a little” solute dissolved

• Adding solute will increase the concentration• Adding water(solvent) will dilute the concentration

Molarity•Amount of substance (mol)•Volume of mixture (L)

•Unit = mol/L or M

Molality• Amount of substance (mol)• Mass of solvent (kg)

• Unit = mol/kg or m

• Volume can change with pressure or temperature (the higher the temp, the bigger the volume), but mass does not change. Sometimes it is better to measure concentration with mass than with volume.

Molality - Example 1• 58.44 grams of NaCl are dissolved in exactly 2.00 kg of pure water.

What would be the molality of the solution? • Molar mass NaCl = 58.44 g/mol

• 58.44 g NaCl = 1 mol NaCl • 1 mol NaCl = 0.5 m • 2.00 kg H2O

Molality – Example 2• A jar of Kool-aid powder recommends pouring a scoop of powder into

2 quarts of water (1.89 kg, 1.98 L). A scoop of drink powder usually weighs 17 g. What is the molality of the resulting Kool-aid?• Kool-aid powder is mostly sugar. We’ll say that the molar mass of

drink powder is the same as the molar mass of sugar: 342 g/mol.

• 17 g Kool-aid | 1 mol = 0.049 mol = 0.026 m 342 g 1.89 kg

Parts per MillionParts per million (ppm)is the ratio of mass units of solute to mass units of solution, multiplied by one million (106)

This is used when there are small amounts of solute in a solvent (like pollution in the air and water)

Click on the “Parts per Million” title to see a helpful video. (PPT must be in play mode.)

Solubility (Notes p.7) •Solubility is the ability of a solute to dissolve in the solvent.

“Like Dissolves Like”

Fat Benzene Steroids Hexane Waxes Toluene

Inorganic Salts Water Sugars Small alcohols Acetic acid

Polar and ionic solutes dissolve best in polar solvents

Nonpolar solutes dissolve best in nonpolar solvents

Soluble means a solvent can dissolve a solute

Another lab• Chemistry Core Curriculum: • Objective 1: Describe factors affecting the process of

dissolving and evaluate the effects that changes in concentration have on solutions.• d. Design and conduct an experiment to determine the

factors (e.g., agitation, particle size, temperature) affecting the relative rate of dissolution.

Factors that affect how fast the solute dissolves

• 1. Particle size / Increasing the surface area (Crushing up solids)• Hypothesis?

• 2. Agitation (Stirring the solution)• Hypothesis?

• 3. Heating up the solvent• Hypothesis?