CH 16- Solutions

Outline Concentration % by mass Molarity Dilution Formula Colligative Properties Concept- explain how it occurs BP Elevation FP Depression

Algebra Review A x D =E B+C

14.3 Concentrations of Solutions Dilute vs. Concentrated? Dilute: a small amount of solute in a large amount of solvent. Concentrated: a large amount of solute in a small amount of solvent. Do not confuse with saturated and unsaturated. For example, “a saturated solution may be either dilute or concentrated” Not very useful terms in science, solutions have to be able to be duplicated exactly to be useful. Ex: strong vs. weak coffee – both are relative to the taster.

% by Mass of a Solute in Solution mass of solute x 100= % by mass mass of solution Calculate the percent by mass of NaCl: 1) If 25 g dissolves to make 150 grams of solution 2) If 40g dissolves in 125 grams of water.

% mass General Formula  mass of solute x 100= % by mass mass of solution 3)How many grams of solute are needed to make 2000g of a 15% by weight solution? 4) How many grams of water are needed to make a 10% solution using 25g of NaCl?

Worksheet 3

Molarity A method used to calculate concentration. Molarity (M) = moles solute L of solution Note: If given grams, use the p-table to find the number of moles When you talk about a solution with a label of 6 M HCl, we say, “ Six molar solution.”

Calculate Molarity 1) 3.6 moles of NaCl in 1.75L 2) 2.4 moles of NaBr in 500mL 3) 500grams of NaCl in 3000ml

How many moles are solute are needed to make…… 1.0.2L of 0.5M Na 2 SO mL of 1.75M KCl

How many grams of solute are needed to prepare 1.4.5L of 6M HCl 2.750mL of 3M H 2 SO 4

Wks 1

What is the relationship between concentration and volume? Proportional or inversely proportional? So what’s the equation?

Dilution Formula

Ex1: What is the molarity of a solution made by adding 35.0 ml of a 6.00 M HCl solution to ml of water?

Ex2: How many ml of a solution containing grams of NaCl in ml of solution would be needed to prepare ml of a M solution? First, calculate the molarity of the original solution: convert g NaCl to moles Now, use the dilution formula to calculate the ml of NaCl solution needed:

If I add 25 mL of water to 125 mL of a 0.15 M NaOH solution, what will the molarity of the diluted solution be? How much 0.05 M HCl solution can be made by diluting 250 mL of 10 M HCl?

Wks 2

Why does freezing point depression occur? The solute (NaCl) interferes with crystal formation. (ex: antifreeze) As the number of solute particles increase, the freezing point decreases.

Boiling point elevation Which would produce more vapor? Which would have a higher vapor pressure? Which would take less energy to raise the vapor pressure to atmospheric pressure? Which would have a higher boiling point?

Boiling point elevation Same concept as freezing point depression except boiling point increases. Why does boiling point elevation occur? The solute takes up space on the surface of a liquid. This decreases the ability of the liquid to evaporate. Thus, the vapor pressure decreases. Boiling occurs when the atmospheric pressure equals the vapor pressure. So, an increase in energy is needed to increase the vapor pressure to reach the atmospheric pressure.

NIB Ion Pairing When experiments are done regarding freezing point depression and boiling point elevation, the actual answers are different than the theoretical answers (surprise, surprise!). Example: a solution of NaCl in water:

NIB Ion Pairing Sodium Chloride can dissociate at a rate of 100% if the concentration of the solution is very low. With increased concentration, ions may come in contact with each other and rejoin resulting in less than 100% dissociation. Only at low, low concentrations do solutions have their “x” factor approach the theoretical value. Draw picture of ion pairing here Theoretical:Actual: X = 14 “ions” X = 10 “ions”

14.4 Colligative Properties of Solutions Solutions that conduct electricity contain electrolytes. Ionic compounds dissociate: NaCl (s) + H 2 O (l) yields Na + (aq) + Cl - (aq) MgCl 2(s) + H 2 O (l) yields Mg +2 (aq) + 2Cl - (aq) Acids ionize (dissociation of a covalent compound): HCl (g) + H 2 O (l) yields H + (aq) + Cl - (aq) H 2 SO 4(l) + H 2 O (l) yields 2H + (aq) + SO 4 -2 (aq)

Substances that are not acids, bases, and salts do not dissociate/ionize When solutes dissolve in liquids, they lower the freezing point. Two factors affect the degree of change in the temperature: the amount of the solute and the nature of the solvent. Colligative properties: a property that depends on the concentration of the solute particles. Freezing point and boiling point are colligative properties.