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
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
8 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
100mL
100 g of water = 100 mL, and the
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
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)
mole
L
x (0.3 L) = 26.64 g
• Make 250 mL of a 3 molar NaCl solution
58.44g x 3 moles
1 mole
1L
? = 43.8g of NaCl
BTV of 250 mL
x 0.25 L = ? g
“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.