Solutions and Concentration Many times in the

Solutions and Concentration • Many times in the lab we work with volumes of solutions in m. L or L instead of weighing solids out in g. • So we talk about the concentration of a substance dissolved or mixed in solution. 1

Solutions and Concentration • First, definitions: • Solvent: component in solution which is present in greatest quantity; everything else is mixed in it. – Solute: substance in solution which is not present in greatest quantity; may be more than 1 solute; it is mixed in the solvent. • What’s the solute and solvent: 25 g benzene is mixed with 30 g ethanol. 2

Solutions and Concentration • One of the most important ways to express concentration is called molarity, or M. • Molarity is defined as: 3

Molarity • Note that we usually drop the solute and sln from the units, but remember what they are! • Note also that molarity is also a conversion factor, converting between mol of solute and L of solution! 4

Molarity • Problems: 5

Molarity • In the lab, we also make a lot of dilutions of solutions. • For example, we often buy stock solutions from chemical suppliers. These are premade solutions whose molarity is known (sort of, usually only to 1 or 2 sig figs). 6

Molarity • But what if we don’t want to use a sln that is that concentration? Well, to make a less concentrated sln we dilute the stock sln. • To do this, we take out a measured volume or the stock sln (called an aliquot); put it in a volumetric flask; and then dilute to the mark with the solvent. 7

Molarity • But first we have to do some calculations as we need to figure out exactly how many m. L of the stock sln we need. • We know the desired new molarity. • We know how many m. L of the new diluted sln we need. 8

Molarity • This is actually a very easy calculation with a very simple equation: M 1 V 1 = M 2 V 2. • It is sometimes expressed as Mc. Vc = Md. Vd where c is the concentrated sln, and d is the dilute sln. 9

Molarity • Common Problem: How many m. L of a 0. 250 M sln of benzene are required to make 250. 00 m. L of a 12. 5 m. M sln? 10

Solution Stoichiometry 11

Solution Stoichiometry • Look at the following rxn: Ag. NO 3(aq) + Na. Cl(aq) Ag. Cl(s) + Na. NO 3(aq) • If I mix 25. 00 m. L of a 0. 100 M silver nitrate sln with plenty of sodium chloride sln, how many g of silver chloride may be made? 12

Solution Stoichiometry • This is just a stoichiometry problem where instead of starting with g of silver nitrate, I start with a known volume of a known molar sln of silver nitrate! • The rest is the same. 13

Solution Stoichiometry Problems • If you mix 25. 00 m. L of a 0. 101 M silver nitrate sln with 35. 00 m. L of a 0. 0989 M sln of Na. Cl, how many g of silver chloride can be produced? • What is the molarity of a silver nitrate sln if 35. 01 m. L are required to react with 15. 89 m. L of a 0. 375 M Na. Cl sln? 14

Solubility of Ionic Salts • How do ionic salts dissolve in water? • It is a surface phenomenon which we’ll go into more detail later. 15

Solubility Rules • Not all ionic compounds are soluble in water. • If they don’t dissolve easily, they are either insoluble (almost none dissolves) or they are slightly soluble (some dissolves). • Slightly soluble salts are also called moderately soluble. 16

Solubility Rules • Why is solubility important? • For now, it helps us determine whether a substance is a strong electrolyte, a weak electrolyte, or a nonelectrolyte. • It also will help us to predict whether a double displacement rxn will occur. • So we need solubility rules! 17

Solubility Rules Soluble Compounds Nitrates Chlorates Perchlorates Exceptions None K is slightly soluble (ss) Acetates Cl-, Br-, I- Ag, Hg(I) are ss Ag, Pb(II), Hg(I) are insoluble (ins) Fluorides Sulfates Ca, Mg, Sr, Ba, Pb(II) are ins Sr, Ba, Pb(II), Hg(I) are ins; Ag, Ca are ss KCl. O 4 None Alkali Metals NH 4+ 18

Solubility Rules Insoluble Compounds Carbonates Phosphates Chromates Exceptions Group I, NH 4+ Oxalates Sulfides Group I, NH 4+, Ca, Sr, Ba Oxides Hydroxides Li, Na, Ca, Sr, Ba Group I, NH 4+, Sr, Ba; Ca is ss 19

Solubility Rules • Which of the following is soluble? Al. PO 4 Mg. SO 4 Na 2 S Fe(OH)3 20

Electrolytes in Aqueous Solution • In order for an electrical current to be conducted, there are 2 requirements: • Moving particles • Charged particles 21

Electrolytes in Aqueous Solution • Electrolytes: Solutes which break apart into ions in water and thus conduct a current. • We also say that these solutes “ionize” in water. 22

Electrolytes in Aqueous Solution • We separate solutes into 3 classes of electrolytes: § Strong: solutes which ionize 100% and so conduct a strong current. § Soluble ionic compounds and strong acids are strong electrolytes. 23

Electrolytes in Aqueous Solution § Weak: solutes which only partially ionize in water and so conduct a weak current. § Weak acids and bases, and a few ionic compounds are weak electrolytes. § The double arrow means that the rxn does not occur 100% (so is not a completion rxn), but only occurs slightly. § It also means that the rxn is reversible, it goes both ways! 24

Electrolytes in Aqueous Solution § Nonelectrolytes: solutes which do not ionize in water and so do not conduct a current. § Insoluble ionic compounds, insoluble acids and bases, and other soluble and insoluble molecular compounds like water, acetone, and methane. 25

Electrolytes in Aqueous Solution § How do you determine whether a compound is a strong, weak, or nonelectrolyte? 1. You memorize the ionic solubility rules. (Note: the strong bases you will work with are ionic hydroxide salts like Na. OH, so they fall into this rule. ) These are strong electrolytes. 2. You memorize the 7 common strong acids: HCl, HBr, HI, HCl. O 4, HNO 3, H 2 SO 4, HCl. O 3. These are strong electrolytes. 3. All other acids and bases are weak electrolytes unless they state it is insoluble in water. 4. All others are nonelectrolytes. 26

Electrolytes in Aqueous Solution § Classify the following as strong, weak, or nonelectrolytes: Na. OH Pb(OH)2 Fe. S (NH 4)2 CO 3 Ca(OH)2 Citric acid (soluble) HNO 2 Sucrose Nitrogen gas Triethyl amine (a soluble base) 27

Ionic Equations & Net Ionic Equations • If some acids, bases, and ionic compounds ionize in water to produce ions, shouldn’t we show this somehow in our chemical equations? • When we write an equation showing ions, it is called an ionic equation. If “spectator” ions are canceled out, it is a net ionic equation. • Here’s an example: A barium chloride sln reacts with a silver nitrate sln to produce a silver chloride ppt and a barium nitrate sln. 28

Equations & Net Ionic Equations Ionic • Your turn: Complete the following rxn in aqueous sln, and write the net ionic equation. (NH 4)2 CO 3(aq) + Zn. Cl 2(aq) --> Zn. CO 3(s) + NH 4 Cl (aq) 29