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, μg1g = 1000mg = 1,000,000µg

• Volume is measured in liters, mL, μL1L = 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

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)? _____ 

8 mL

5 mg = ?8 mL 1 mL

? = 0.63 mg, so the concentration is 0.63 mg/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 100mLsolution 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

• 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 distilled 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.

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. – 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

“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% bleachBTV 350 mL.