PREPARING SOLUTIONS AND REAGENTS

Chemical Solutions (aqueous = water is the solvent) Types of vessels (least to most precise): Beaker Erlennmeyer flask Graduated cylinder Volumetric flask

DEFINITIONS: SOLUTES -- substances that are dissolved SOLVENTS -- substance in which solutes are dissolved (usually water) AMOUNT -- how much

Goals Make solutions Dilute solutions Convert between different concentrations of solutions

Facts of Life Mass is measured in Grams, mg, μg 1g = 1000mg = 1,000,000µg Volume is measured in liters, mL, μL 1L = 1000mL = 1,000,000 μL Density of water is 1 g / mL

Facts of life (cont’d) Concentration means: amount of solute in a volume of solution Expressed in many ways: 1. percent 2. mg/ mL 3. molar 4. “X” solution

? = 0.63 mg, so the concentration is 0.63 mg/mL Each star represents 1 mg of NaCl. What is the total amount of NaCl in the tube? _____ What is the concentration of NaCl in the tube (in mg/mL)? _____   5 mg = ? 8 mL 1 mL ? = 0.63 mg, so the concentration is 0.63 mg/mL 8 mL

Percent Solutions Per means “for every one” Cent means 100 Example: a 5% sugar solution has 5 grams of sugar for 100g of solution, or 5g 100 g of water = 100 mL, and the 100mL solution is mostly water.

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 distilled water

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

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

“X” solutions X means times A 40X buffer solution is 40 times more concentrated than the standard working solution Stock solutions / concentrates How much stock solution you need = total volume you need divided by the “X” number

Diluting Solutions Conc 1 x Vol. 1 = Conc 2 x Vol 2 Usually want 1 X solutions

Example: Frozen Orange Juice Solution 1 is the frozen concentrate Solution 2 is the 1X juice you drink How concentrated is it? C1 V1 = C2 V2 (? X) (250mL) = (1X) (1000 mL)

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

Example: bleach sterilant Solution 1 is 100% bleach – stock solution Solution 2 is 6% bleach – what you want How do you make 350 mL of 6% bleach? C1 V1 = C2 V2 (100%) (?) = (6%) (350 mL)

Answer: You need 21 mL of 100% bleach BTV 350 mL.

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.

MOLARITY Molarity is: number of moles of a solute that are dissolved per liter of total solution. A 1 M solution contains 1 mole of solute per liter total volume.

MOLE How much is a mole?

EXAMPLE: SULFURIC ACID For a particular compound, add the atomic weights of the atoms that compose the compound. H2SO4: 2 hydrogen atoms 2 X 1.00 g = 2.00 g 1 sulfur atom 1 X 32.06 g = 32.06 g 4 oxygen atoms 4 X 16.00 g = 64.00 g 98.06 g

EXAMPLE CONTINUED A 1M solution of sulfuric acid contains 98.06 g of sulfuric acid in 1 liter of total solution. "mole" is an expression of amount "molarity" is an expression of concentration.

DEFINITIONS "Millimolar", mM, millimole/L. "Micromolar", µM, µmole/L. A millimole is 1/1000 of a mole. "Micromolar", µM, µmole/L. A µmole is 1/1,000,000 of a mole.

FORMULA HOW MUCH SOLUTE IS NEEDED FOR A SOLUTION OF A PARTICULAR MOLARITY AND VOLUME? (g solute ) X (mole) X (L) = g solute needed 1 mole L or FW X molarity x volume = g solute needed

EXAMPLE How much solute is required to make 300 mL of 0.8 M CaCl2?

ANSWER (111.0 g) x (0.8 mole) x (0.3 L) = 26.64 g mole L

Make 250 mL of a 3 molar NaCl solution 58.44g x 3 moles x 0.25 L = ? g 1 mole 1 L ? = 43.8g of NaCl BTV of 250 mL