Chapter 14 Solutions

Solutions Solutions are homogeneous mixtures of two or more pure substances. (remember this from chapter 3) In a solution, the solute is dispersed uniformly throughout the solvent.

Aqueous solution-use water as solvent (recall chapter 7) Dilute solution- contains a small amount of solute Concentrated solution- contains large amount of solute

How Does a Solution Form: Chemists use the axiom “like dissolves like”: Polar substances tend to dissolve in polar solvents. Nonpolar substances tend to dissolve in nonpolar solvents.

How Does a Solution Form: chemical explanation The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.

How Does a Solution Form: chemical explanation As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

How Does a Solution Form: chemical explanation The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

How Does a Solution Form: chemical explanation Glucose (which has hydrogen bonding) is very soluble in water (which has hydrogen bonding), while cyclohexane (which only has dispersion forces) is not.

it doesn’t mean the substance dissolved. Student, Beware! Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved.

Student, Beware! Dissolution is disappearance of a substance into solution Dissolution is a physical change—you can get back the original solute by evaporating the solvent. If you can’t, the substance didn’t dissolve, it reacted.

Types of Solutions

Types of Solutions Saturated Solvent holds as much solute as is possible at that temperature. Dissolved solute is in dynamic equilibrium with solid solute particles.

Types of Solutions Unsaturated Less than the maximum amount of solute for that temperature is dissolved in the solvent.

Types of Solutions Supersaturated Solvent holds more solute than is normally possible at that temperature. These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask. Example: ornament experiment

Solubility Charts

WhY do you keep highlighting Temperature? Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

WhY do you keep highlighting Temperature? The opposite is true of gases: Carbonated soft drinks are more “bubbly” if stored in the refrigerator. Warm lakes have less O2 dissolved in them than cool lakes.

How does the solubility curve relate to saturation? above the line- supersaturated (holding more than it can) on the line- saturated (can not hold anymore) below the line- unsaturated (can hold more solute) supersaturated Solute (g) per 100 g H2O saturated unsaturated temperature

Ways of Expressing Concentrations of Solutions

Mass Percentage mass of A in solution  100 total mass of solution

Parts per Million and Parts per Billion Parts per Million (ppm) mass of A in solution total mass of solution ppm =  106 Parts per Billion (ppb) mass of A in solution total mass of solution ppb =  109

Mass Percent sample problem A solution is prepared by dissolving 1.0g of sodium chloride in 48 g of water. The solution has a mass of 49 g, and there is 1.0g of solute (NaCl) present. Find the mass percent of solute.

Sample problem #2 A solution is prepared by mixing 1.00g of ethanol, C2H5OH, with 100.0 g of water. Calculate the mass percent of ethanol in this solution.

Molarity

Molarity molarity is the concentration of a solution given in gram moles of solute per liter of solution. Ex. A 5M solution of hydrochloric acid has a higher concentration of hydrochloric acid than a 0.2M solution The higher the molarity in this case the more caution needed.

Molarity (M) mol of solute M (mol/L)= L of solution Because volume is temperature dependent, molarity can change with temperature, but we will save those calculations for AP chemistry

Molarity = 0.192 M NaOH 11.5 g NaOH x 1 mol NaOH 40.0 g NaOH Convert mass of solute to moles (using molar mass of NaOH). Then we can divide by volume molar mass of solute = 40.0 g 11.5 g NaOH x 1 mol NaOH 40.0 g NaOH 0.288 mol NaOH 1.50 L solution = 0.288 mol NaOH = 0.192 M NaOH

Molarity Calculate the molarity of a solution prepared by dissolving 1.56 g of gaseous HCl into enough water to make 26.8 mL of solution. Given: mass of solute (HCl) = 1.56 g volume of solution = 26.8 mL

Molarity Calculate the molarity of a solution prepared by dissolving 11.5g of solid NaOH in enough water to make 1.50 L of solution. Given: mass of solute = 11.5 g NaOH vol of solution = 1.50 L molarity is moles of solute per liters of solution

dilution

dilution Diluting a solution: reduces the number of moles of solute per unit volume the total number of moles of solute in solution does not change

M1 = molarity of stock solution (initial) Diluting solutions M1V1 = M2V2 M1 = molarity of stock solution (initial) V1 = volume of stock solution (initial) M2 = molarity of dilute solution V2 = volume of dilute solution

M1V1 = M2V2 How many milliliters of aqueous 2.00M MgSO4 solution must be diluted with water to prepare 100.00 mL of aqueous 0.400M MgSO4? M1 = 2.00M MgSO4 M2 = 0.400M MgSO4 V2 = 100.00 mL MgSO4 V1 = ?

M1V1 = M2V2 Solve for V1 V1 = M2 x V2 M1 0.400M x 100.00 mL 2.00M