GSCI 163

Lecture 10

Solutions

• How do we measure solubility?

There are 3 different ways of doing.1. Molarity: The number of moles of solute per liter

of solvent2. Weight percentage: %w or w/w3. Parts per million (ppm) or parts per billion (ppb)

Molarity

Molarity = number of moles solute/volume of solvent

• What is the molarity of a solution containing 0.32 moles of NaCl in 3.4 liters of water?

Examples

• What is the molarity of a solution made by dissolving 2.5 g of NaCl in enough water to make 125 ml of solution?

Example

• How would you prepare 400. ml of 1.20 M solution of sodium chloride?

Weight Percentage

• What is the percentage weight concentration of a solution made by dissolving 2.5 g of NaCl in enough water to make 125 ml of solution? (1ml water = 1g of water)

Why sugar dissolves well in water?• Table sugar dissolves in water because when a sucrose molecule breaks

from the sugar crystal, it is immediately surrounded by water molecules. The sucrose has hydroxyl groups that have a slight negative charge. The positive charge of the oxygen found in the water molecule binds with the sugar. As the hydration shell forms around the sucrose molecule, the molecule is shielded from other sugar molecules so the sugar crystal does not reform.

Acids and bases

Dissociation of water

H2O + H2O H3O+ + OH-

The reaction happens in both direction. The question is, how many of molecules of water are dissociated at any time?

Concentration of ions

The answer is: 1 in every 560 million.

Then we can calculate the concentration ions in equilibrium

Kw = [H3O+][OH-] = 10-14 molarity of dissociated water molecules

Concentration of [OH- ] = 10-7 [moles per liter]Concentration of [H3O+] = 10-7 [moles per liter]

What happens if one adds more hydroxide in solution?

Changing the equilibrium concentration

Answer: more OH- will combine with hydroniuns and its concentration will decrease.

OH- + H3O+ H2O

What happens if we increase the concentration of “protons” (H+)?

Definition of acid and base

• Acid is a proton donor – any substance that when dissolved in water increase the number of hydronium ion concentration

HCl + H2O H3O+ + Cl-

• Base is a proton receptor – any substance that when dissolved in water increases the hydroxide concentration

NH3 + H2O (NH4)+ + OH-

pH scale

The pH scale is the concentration of hydonium ions in solution.

Strong acids, ionize completely in water, whereas weak acids only partially ionize in water.

Same is true for bases.

Next class

• More on acids and bases