Chemical Calculations for Solutions Aqueous Solutions Much of

Chemical Calculations for 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 wide array of sizes, and are marked to indicate a specific volume of solution. • Ex. 250 m. L of 1. 43 M (NH 4)2 Cr 2 O 7 (aq) is prepared by adding the appropriate mass of the salt to a 250 m. L volumetric flask and filling up to the mark.

Examples • 0. 5 g of Cobalt (II) chloride are dissolve in enough H 2 O to produce 10 m. L of solution. What is the molarity of Co. Cl 2 ? • Consider the reaction below. How many m. L of the Co. Cl 2 (aq) solution would be needed to completely react 2 g of Na 2 S(s)?

Dilution • In many instances (especially in lab), you may need to prepare a solution of some desired concentration 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 cap-full or so (aliquot) to a large volume of water to attain a manageable solution. • 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. • We know that the moles (n) of solute in V liters of a solution with molarity M is: n = MV • The moles of solute present before addition of water (n 1) must be same as the moles of solute present after (n 2) • Therefore: V 1 V 2

How to perform a Dilution Take an aliquot (V 1) of the stock solution, add it to a new container High concentration stock solution of concentration M 1 Aliquot of stock Dilute with solvent solution with volume to desired volume, V 1 and concentration V 2 M 1. After mixing, we have a dilute solution with volume V 2 and concentration 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 instead prefer to have 1 L of a 1. 0 M solution. How would you prepare this? We are given: • initial concentration of the Na. OH stock (M 1 = 19. 1 M), • the final 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) • A 52 m. L aliquot of the stock solution is added to 948 m. L of water to make 1 L of a 1. 0 M solution.

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

Applying Molarity to Stoichiometric Calculations • 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 2 (L) formed. Limiting Reactant !