I. Making Solutions

A. Background

Definitions

Solution - homogeneous mixture of one or more substances dissolved in another

Solute - the substances (solids or liquids) that are dissolved

Solvent - the "dissolver" - a liquid, usually water

A. The procedure for making solutions

Kitchen: Fruit Punch Lab: 50X Denhardt's Solution

Ingredients:

12 oz frozen pineapple juice 5 g Ficoll

20 oz frozen strawberries 5 g Polyvinylpyrrolidone

1 pint strawberry ice cream 5 g BSA (fraction V)

Chilled lemon-lime soda Nuclease-free water

Preparation:

1. Combine each in turn, Stir w/heating, dissolving

blending until smooth: each solute completely in turn:

pineapple juice concentrate Ficoll

strawberries PVPP

ice cream BSA

2. Pour into chilled punch bowl Transfer to graduated cylinder

3. Add soda to 2 quarts Bring to 500mL with water

4. Top with ice cream Filter through a 0.22 mm filter

5. Serve in punch cups. Store in 50mL pre-sterilized tubes.

B. Many recipes for lab solutions do NOT give amounts but instead give concentrations:

Amounts: Concentrations: (amount per volume)

5g g/L (mg/ml)

500mL %

M (mM)

2. Making solutions to concentration

a. Molarity - number of moles of solute dissolved per liter of solution (moles/L)

Review:

1 mole of any element contains 6.02 x 10²³ atoms

*the weight of 1 mole of an element is its gram atomic weight

Ex. 1 mole of C weighs 12.0g

Compounds are atoms of 2 or more elements bonded together

1 mole of a compound contains 6.02 x 10²³ molecules

*the weight of 1 mole of a compound is its gram formula weight (FW) or gram molecular weight (MW)

Ex. 1mole of Na₂SO₄ is 142.04g:

2 Na atoms = 2 x 22.99g = 45.98g

1 S atom = 1 x 32.06g = 32.06g

4 O atoms = 4 x 16.00g = 64.00g

142.04g

A 1 molar (1M) solution of a compound contains 1 mole of that compound dissolved in 1 L of total solution.

Ex. 1M Na₂SO₄ = 1mole or 142.04g of Na₂SO₄ in 1L of solution

Ex. 1mM Na₂SO₄ = 1mmole (10^-3M) or 0.14204g of Na₂SO₄ in 1L of solution

Ex. 1mM Na₂SO₄ = 1mmole (10^-6M) or 0.000142g of Na₂SO₄ in 1L of solution

Making 1L of 1M Na₂SO₄ - MW = 142.04g/mol

Making 100mL of 1M Na₂SO₄

Making 100 mL of 10mM Na₂SO₄

b. Weight per Volume - solute expressed as weight, solvent as volume

Ex. 2mg/mL proteinase K

each mL solution contains 2 mg Proteinase K

You want to make 50 mL of Proteinase K solution at a concentration of 2 mg/mL

You know the concentration and volume of solution you want, you need to figure out the amount of solute needed:

c. Percent - amount of solute in 100 units of total solution

What is 2% milk?

2g of milk solids per 100mL of milk

a weight per volume expression (w/v)

i. Weight per volume

Ex. 20g NaCl in 100mL total solution is a 20% (w/v) solution

ii. Volume percent

Ex. 100mL of methanol in 1000mL of total solution is a 10% (v/v) solution

2. Preparing dilute solutions from concentrated solutions

Directions for making orange juice from concentrate:

Add 1 can concentrated orange juice to 3 cans cold water.

How can we express the concentration of our “drinkable” orange juice?

How can we express the concentration of our concentrated orange juice?

Concentration_Stock x Volume_Stock = Concentration_Final x Volume_Final

C₁V₁ = C₂V₂

C₁ = initial concentration of stock solution

V₁ = volume of stock solution required

C₂ = final concentration desired

V₂ = final volume desired

Want to prepare 500mL of 1X TBE from a 5X stock

C₁V₁ = C₂V₂

What is known?

What is the unknown?

Want to prepare 100 mL of 1M Tris from a 2M Tris stock solution.

C₁V₁ = C₂V₂

What is known?

What is the unknown?

3. Preparing solutions with more than one solute

preparing without stocks

preparing with stock solutions - use the C₁V₁ = C₂V₂ equation for each solute