Section 6.2—Concentration How do we indicate how much of the electrolytes are in the drink? Objectives: Calculate concentrations in percent concentration and molarity Calculate the concentration of electrolytes in solutions of ionic compounds

Concentrated versus Dilute solvent solute Lower concentration Not as many solute particles Higher concentration More solute particles

Concentration Concentration gives the ratio of amount of solute dissolved to total amount of solute and solvent There are several ways to show concentration

Percent Mass/Volume This is a method of showing concentration that is not used as often in chemistry However, it’s used often in the food and drink industry For example, your diet drink can might say you have less than 0.035 g of salt in 240 mL. That would give you a concentration of 0.035 g / 240 mL, which is 0.015% solution

%(M/V) Example Example: If you dissolve 12 g of sugar in 150 mL of water, what percent mass/volume is the solution?

%(M/V) Example 8.0% (M/V) Example: If you dissolve 12 g of sugar in 150 mL of water, what percent weight/volume is the solution? 8.0% (M/V)

Practice Problems Determine the % M/V of a carbohydrate in a sports drink if it contains 14 g in 240 mL; if it contains 20 g in 240 mL. A certain sports drink contains 0.050 g sodium chloride in 240 mL. What is its concentration in % M/V? What if it had 0.100 g in 240 mL?

Concentration using # of molecules When working with chemistry and molecules, it’s more convenient to have a concentration that represents the number of molecules of solute rather than the mass (since they all have different masses) Remember, we use moles as a way of counting molecules in large numbers

Mole Review The molar mass of a substance is found by adding up all the atomic masses in the substance Example: The molar mass of NaCl is 58.5 g The molar mass of a substance in grams represents 1 mole of that substance. 1 mole of NaCl has a mass of 58.5 g

Practice Determine the molar mass of each of the following substances. Ca(OH)2 Mg C2H3OH Cu(NO3)2

Review: Calculating # of Moles If you are given the mass of a substance, you use the molar mass to determine the number of moles. Example how many moles are in 35 g of Mg? 35 g x 1 mole = 1.4 moles 24.3 g

How many moles are in 25.5 g NaCl? Practice: How many moles are in 25.5 g NaCl?

How many moles are in 25.5 g NaCl? Example: How many moles are in 25.5 g NaCl? Na Cl 1 23.0 g/mole 35.5 g/mole  = + 58.5 g/mole 1 mole NaCl molecules = 58.5 g 25.5 g NaCl 1 mole NaCl = _______ mole NaCl 0.436 58.5 g NaCl

More Practice Determine the number of moles in each of the following: 68.0 g CaCl2 9.2 g CH4 3. 70.1 g Al(OH)3

Molarity Molarity (M) is a concentration unit that uses moles of the solute instead of the mass of the solute

Molarity Example Example: If you dissolve 12 g of NaCl to make 150 mL of solution, what is the molarity?

Next, remember to change mL to L! Divide by 1000. Molarity Example Example: If you dissolve 12 g of NaCl to make 150 mL of solution, what is the molarity? Na Cl 1 23.0 g/mole 35.5 g/mole  = + 58.5 g/mole 1 mole NaCl molecules = 58.5 g 12 g NaCl 1 mole NaCl = _______ mole NaCl 0.21 58.5 g NaCl Next, remember to change mL to L! Divide by 1000. 150 mL of water = 0.150 L = 1.4 M NaCl

Practice Problems Determine the molarity of 2.5 L of solution made from 125 g of AgNO3. What is the molarity of a solution produced when 145 g of NaCl is dissolved in enough water to make 2.75 L of solution? What is the molarity of a solution in which 85.6 g of HCl are dissolved in enough water to make 385 mL of solution? 8.77 g of KCl is dissolved in sufficient water to make 4.75 L of solution. Determine the molarity of the solution.

Converting between the two If you know the %(W/V), you know the mass of the solute You can convert that mass into moles using molecular mass You can then use the moles solute to find molarity

Converting from % to M Example What molarity is a 250 mL sample of 7.0 %(W/V) NaCl?

Converting from % to M Example What molarity is a 250 mL sample of 7.0 %(W/V) NaCl? ? = 17.5 g NaCl Na Cl 1 22.99 g/mole 35.45 g/mole  = + 58.44 g/mole 1 mole NaCl molecules = 58.44 g 17.5 g NaCl 1 mole NaCl = _______ mole NaCl 0.30 58.44 g NaCl Remember to change mL to L! 250 mL of water = 0.250 L 1.2 M NaCl

Concentration of Electrolytes An electrolyte breaks up into ions when dissolved in water You have to take into account how the compound breaks up to determine the concentration of the ions CaCl2  Ca+2 + 2 Cl-1 For every 1 CaCl2 unit that dissolves, you will produce 1 Ca+2 ion and 2 Cl-1 ions If the concentration of CaCl2 is 0.25 M, the concentration of Ca+2 is 0.25 M and Cl-1 is 0.50 M

Let’s Practice #1 Example: You want to make 200 mL of a 15% (W/V) solution of sugar. What mass of sugar do you need to add to the water?

Let’s Practice #1 30 g of sugar Example: You want to make 200 mL of a 15% (W/V) solution of sugar. What mass of sugar do you need to add to the water? 30 g of sugar

What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution? Let’s Practice #2 Example: What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution?

What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution? Let’s Practice #2 Example: What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution? ? = 0.125 moles CaCl2 Ca Cl 1 2 40.08 g/mole 35.45 g/mole  = 70.90 g/mole + 110.98 g/mole 1 mole CaCl2 molecules = 110.98 g 0.125 moles CaCl2 110.98 g CaCl2 = _______ g CaCl2 13.9 1 mole CaCl2 2.8 %(W/V) CaCl2

Let’s Practice #3 Example: What are the molarities of the ions made in a 0.75 M solution of Ca(NO3)2

Let’s Practice #3 Ca(NO3)2  Ca+2 + 2 NO3-1 Ca+2 = 0.75 M Example: What are the molarities of the ions made in a 0.75 M solution of Ca(NO3)2 Ca(NO3)2  Ca+2 + 2 NO3-1 For every 1 Ca(NO3)2, there will be 1 Ca+2 and 2 NO3-1 ions Ca+2 = 0.75 M NO3-1 = 1.5 M