Solution Reactions

# Solution Reactions - Week 3 Composition of Solutions Mass...

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Week 3 Composition of Solutions Mass Percentage (weight percentage) of a solution o Mass of certain substance divided by the total mass of all substances in solution Mole fraction o Number of moles of substance / total moles present o X 1 = n 1 / (n 1 + n 2 ) , X 2 = n 2 /(n 1 +n 2 ) = 1-X 1 Concentration o Molarity : moles solute/liters solution (units: moles/liter) 0.3 M NaCl solution has 0.3 moles of NaCl per liter of solution (water, for instance) o Molality : moles solute/kilograms solvent (units: mol/kg) Example Problem: o You are given 52 grams of MgCO 3 and 3 L of water. Assuming that when you add the magnesium carbonate to the water the volume and density do not change, calculate the following: Mass percentage of Mg, C, and O in the solution Molarity of Mg 2+ , CO 3 2- , and MgCO 3 Molality of Mg 2+ , CO 3 2- , and MgCO 3 in water (density 1g/1mL) Preparing Solutions o We know that when we add solids to liquids, the volume does increase. In order to calculate this increase we use the following equation: c f = moles solute / final solution volume = c i V i /V f c = molarity or concentration measurement o if we do not need to do dilutions but just prepare a solution, we use a volumetric flask, add LESS volume of the solvent than needed, then add the solvent, and finally fill until the final volume desired is reached. Nature of Dissolved Species During dissolution, the molecules in their original phases (solid, liquid or gas) are broken up and replaced by new solute-solvent attractions Water is one of the most common solvents (especially for inorganic chemistry) so the book and class will be focused on aqueous (water based) solutions Remember that water is an extremely polar solvent, and therefore the solute-solvent interactions must involve some polarity or induced polarity in order to mix with the water Aqueous Solutions of Ionic species (electrolytes) o In solutions, ionic compounds “break up” into their ionic components i.e. Mg 2+ , CO 3 2- break up in the water solution above

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o solvation shell surrounds the ions. If the ion is +, then the oxygens of water will face the ion. If the ion is -, then the hydrogens of water will face the ion. o
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## This note was uploaded on 10/26/2011 for the course CHEM 20B taught by Professor Felker during the Spring '08 term at UCLA.

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Solution Reactions - Week 3 Composition of Solutions Mass...

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