Solutions and Concentration

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

2. 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: 25g benzene is mixed with 30g ethanol.

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

4. 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!

5. Molarity • Problems:

6. 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).

7. 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.

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

9. Molarity • This is actually a very easy calculation with a very simple equation: M1V1 = M2V2. • It is sometimes expressed as McVc = MdVd where c is the concentrated sln, and d is the dilute sln.

10. Molarity • Common Problem: How many mL of a 0.250 M sln of benzene are required to make 250.00 mL of a 12.5 mM sln?

11. Solution Stoichiometry

12. Solution Stoichiometry • Look at the following rxn: AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq) • If I mix 25.00 mL of a 0.100M silver nitrate sln with plenty of sodium chloride sln, how many g of silver chloride may be made?

13. 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.

14. Solution Stoichiometry Problems • If you mix 25.00 mL of a 0.101 M silver nitrate sln with 35.00 mL of a 0.0989 M sln of NaCl, how many g of silver chloride can be produced? • What is the molarity of a silver nitrate sln if 35.01 mL are required to react with 15.89 mL of a 0.375M NaCl sln?

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

16. 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.

17. 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!

18. Solubility Rules

19. Solubility Rules

20. Solubility Rules • Which of the following is soluble? • AlPO4 MgSO4 Na2S Fe(OH)3

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

22. 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.

23. 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.

24. 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!

25. 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.

26. Electrolytes in Aqueous Solution • How do you determine whether a compound is a strong, weak, or nonelectrolyte? • You memorize the ionic solubility rules. (Note: the strong bases you will work with are ionic hydroxide salts like NaOH, so they fall into this rule.) These are strong electrolytes. • You memorize the 7 common strong acids: HCl, HBr, HI, HClO4, HNO3, H2SO4, HClO3. These are strong electrolytes. • All other acids and bases are weak electrolytes unless they state it is insoluble in water. • All others are nonelectrolytes.

27. Electrolytes in Aqueous Solution • Classify the following as strong, weak, or nonelectrolytes: NaOH Pb(OH)2 FeS (NH4)2CO3 Ca(OH)2 Citric acid (soluble) HNO2 Sucrose Nitrogen gas Triethyl amine (a soluble base)

28. 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.

29. Ionic Equations & Net Ionic Equations • Your turn: Complete the following rxn in aqueous sln, and write the net ionic equation. (NH4)2CO3(aq) + ZnCl2(aq) --> ZnCO3(s) + NH4Cl (aq)