Solutions Aqueous Solutions Much of the chemistry that

Solutions

Aqueous Solutions • Much of the chemistry that affects us occurs among substances dissolved in water (proteins, salts, carbohydrates, etc. ) • Solutions are homogenous mixtures, meaning that the components comprising the solution are uniformly dispersed • The most common type of solution is a solid dissolved in a liquid. The dissolved solid is the solute, the liquid is the solvent. • Solutes and solvents do not react, merely co-exist, as is the case with an aqueous solution like salt water Na. Cl (s) -----> Na. Cl(aq) H 2 O (L)

Concentration (Molarity) •

Preparing an Aqueous Solution (Ex. 250 m. L of 1. 43 M Ammonium Dichromate) • Typically, a volumetric flask is used to prepare solutions. Volumetric flasks come in a variety of sizes, and are marked to indicate a specific volume of solution. • For example, 250 m. L of a 1. 43 M ammonium dichromate, (NH 4)2 Cr 2 O 7 (aq) solution is prepared by adding the appropriate mass of the solute to a 250 m. L volumetric flask and filling up to the mark with solvent.

Examples • 0. 5 g of Co. Cl 2 are dissolve in enough H 2 O to produce 10 m. L of solution. What is the molarity of Co. Cl 2 ? • How many moles of the solute would be present in 6. 3 m. L of this solution?

Example • 15 g of Aluminum nitrate, Al(NO 3)3, is dissolved in enough water to produce 200 m. L of solution. What is the molarity of nitrate in the solution? • Aluminum nitrate is a salt, so it will dissociate into aluminum and nitrate ions, as according to the chemical formula: Al(NO 3)3 (aq) Al 3+(aq) + 3 NO 3 -(aq) • Therefore, every mole of aluminum nitrate yields 3 moles of nitrate

Dilution • In many instances (especially in lab), you may need to prepare a working solution from a pre-existing stock solution. • For example, consider a concentrated detergent like Tide®. If wanted to wash a shirt, you wouldn’t just dump Tide® on it. • Instead, you add a few m. L (aliquot) to a large volume of water to attain a manageable concentration of detergent. • This action of “watering down” the detergent to a useable state is called dilution. The bottle of Tide® is the stock solution.

Dilution • Keep in mind that dilution does not change the total moles of solute, only the molarity. V 1 V 2

How to perform a Dilution High molarity stock solution of molarity M 1 Aliquot of stock solution with volume V 1 and molarity M 1 added to new container. Dilute with solvent to desired volume, V 2 MIX. You now have a dilute solution with volume V 2 and molarity M 2

Example • You need to perform an experiment using Na. OH (aq). At your disposal is 1 L of a concentrated stock solution of 19. 1 M Na. OH (aq). This is much too concentrated for your intended purpose. You would prefer to have 1 L of a 1. 0 M solution. How would you perform this dilution? We are given: • initial concentration of the Na. OH stock (M 1 = 19. 1 M), • the desired diluted concentration of Na. OH (M 2 = 1. 0 M), • and the final volume of the diluted solution (V 2 = 1 L). Ø We need to find the volume of the aliquot (V 1) • Add 52 m. L of the stock solution to 948 m. L of water and mix to obtain 1 L of a 1. 0 M Na. OH (aq) solution.

Group Examples • a. ) Explain how would you make a 500 m. L stock solution that is 0. 1 M Sodium Selenide. • b. ) USING THIS STOCK SOLUTION, you decide to perform a dilution to prepare 100 m. L of a 0. 035 M solution. Explain how you would do this. Dilution

Applying Molarity to Stoichiometry • For reactions of solutions, we can use molarity to calculate product yields • Example: Mn. O 2(s) + 4 HBr(aq) -----> Mn. Br 2(aq) + Br 2(L) + 2 H 2 O(L) 3. 62 g of Mn. O 2 is added to 25 m. L of a 0. 85 M HBr(aq) solution. Determine the mass of Br (L) formed. Limiting Reactant !

Group Example • A beaker contains 500 m. L of 0. 34 M nitric acid, HNO 3(aq). A 1. 07 g chunk of silver metal is added to the beaker, and a single replacement reaction proceeds, yielding silver (I) nitrate (aq) and hydrogen gas. Calculate the mass of hydrogen produced, and the molarity of silver (I) nitrate (aq) if the reaction has a 90% yield. You may assume that the change in the volume of the solution following the addition of Ag is negligible.