Section 15.2 Describing Solution Composition 1. To understand mass percent and how to calculate it Objective

Section 15.2 Describing Solution Composition A. Solution Composition: Mass Percent Mass percent = _mass of solute_ x 100 mass of solution = __grams of solute__ x 100 g solute + g solvent Ex.: 1.0 g of sodium chloride is dissolved in 48g water. mass of solution = 1.0g + 48g = 49g mass percent of solute = (1.0g/49g)100 = 2.0% NaCl

Section 15.2 Describing Solution Composition 1. To understand and use molarity 2. To learn to calculate the concentration of a solution made by diluting a stock solution Objectives

Section 15.2 Describing Solution Composition B. Solution Composition: Molarity Concentration of a solution - amount of solute in a given volume of solution

Section 15.2 Describing Solution Composition B. Solution Composition: Molarity n L M Ex.: a 1.0 M solution has 1.0 mole of solute in 1 liter of solution. M = n/L 

Section 15.2 Describing Solution Composition B. Solution Composition: Molarity To find the moles of solute in a given volume of solution of known molarity use the definition of molarity (M = n/L): Ex.: Calculate the molarity of a solution prepared by dissolving 11.5 g NaOH in enough water to make 1.50 L of solution. We need to know how many moles of NaOH the solution contains: 11.5 g NaOH x 1 mol NaOH = mol NaOH 40.0 g NaOH Molarity (M) = moles of solute = mol/L liters of solution M = __0.288 mol NaOH_ = M NaOH 1.50 L solution

Section 15.2 Describing Solution Composition B. Solution Composition: Molarity Standard solution - solution of accurately known concentration To make a standard solution –Weigh out a sample of solute. –Transfer to a volumetric flask. –Add enough solvent to the mark on the flask.

How do we make a solution of exactly the concentration we want? We need to know how much of the solute (powder) or concentrated solution to use, then add enough water (solvent) to make it just right.

Section 15.2 Describing Solution Composition C. Dilution Water can be added to an aqueous solution to dilute the solution to a lower concentration. Only water is added in the dilution – the amount of solute is the same in both the original and final solution. The number of moles of solute is not changed by adding more solvent; the concentration of the new solution can be calculated: M 1 V 1 = n = M 2 V 2 M 1 V 1 = M 2 V 2

Section 15.2 Describing Solution Composition D. Dilution Diluting a solution –Transfer a measured amount of original solution to a flask containing some water. –Add water to the flask to the mark (with swirling) and mix by inverting the flask.