Chemical Solutions (aqueous = water is the solvent)

Types of vessels (least to most precise):

Beaker Erlennmeyer flask Graduated cylinder Volumetric flask

Goals

• Make solutions

• Dilute solutions

• Convert between different concentrations of solutions

Facts of Life

• Mass is measured in Grams, mg, μg1g = 1000mg = 1000000µg

• Volume is measured in liters, mL, μL1L = 1000mL = 1000000 μL

• Density of water is 1 g / mL

Facts of life (cont’d)

• Concentration means: amount of solute in a volume of solution

• Expressed 3 ways:– 1. percent – 2. mg/ mL– 3. molar

Percent Solutions

• Per means “for every one”

• Cent means 100

• Example: a 5% sugar solution has

5 grams of sugar for 100g of solution

Make 250 mL of a 3% starch solution

3 g / 100 g = 3 g / 100 mL because density of water is 1 g / mL

Set up a ratio:

3 g / 100 mL = ?g / 250 mL

Use 7.5 g of starch and bring to a volume of (BTV) 250 mL with water

• 5 mg/mL has 5 milligrams of solute in 1 milliliter of solution

mg / mL Solutions

Set up a ratio:

3 mg / 1 mL = ?mg / 250 mL

Use 750 mg of starch and bring to a volume of (BTV) 250 mL with water

Make a 250 mL of a 3 mg /mL starch solution

Molar Solutions

• 1 mole is 6.02 x 1023 items

• Molecular weight or Formula weight is really the mass of 1 mole of molecules (see periodic table)

Example: 1 mol of sodium chloride (NaCl) has a mass of 58.44 g.

A 1 Molar solution is 1 mol of solute /Liter of solution

• Make 250 mL of a 3 molar NaCl solution

Set up a ratio:

58.44 g /1 mol = ? g / 3 mol

3 moles has a mass of 175.3 g

3M = 3mol/L = 175.3g/1000mL = ? g/250mL

? = 43.8g

BTV of 250 mL

Diluting Solutions

• Conc A x Vol. A = Conc B x Vol B

• Usually want 1 X solutions

Example: Frozen Orange Juice

• Solution A is the frozen concentrate

• Solution B is the 1X juice you drink

• How concentrated is it?

• CA VA = CB VB

• (250mL)(?X) = (1000 mL)(1X)

Answer:

Frozen OJ is 4X because ¼ of the final volume is the concentrated oj