Water and Solutions
The importance of water
• Important component of solutions
• Water quality, composition, and pH can
drastically affect an experiment
Which water should I use?
• Tap water
▫ Variable chemistry and purity
• Laboratory grade water
▫ Reverse osmosis or distillation
▫ Rinsing glassware, media preparation
• Reagent grade water
▫ Filtration, deionization, and carbon adsorption
▫ Most laboratory uses
• Other specialized types (dd, microfiltered,
electrically deionized, etc.)
• Solution: A mixture in which individual
molecules or ions are dispersed in a liquid.
• Solvent: The liquid that makes up the majority
of the solution.
▫ e.g., water (aqueous solution)
• Solute: The minority component of the
solution. Often a solid before mixing.
Measurements of concentration
• In almost all cases, these are amount of solute
per volume of SOLUTION.*
• Weight-per-volume (e.g., mg/L)
• Percent (parts per hundred)
• PPT, PPM, PPB
• Molar solutions
* [The exception is molal solutions (moles of
solute per liter of solvent) but we will not discuss
them further in this class.]
• g/L, µg/mL, etc.
• How to make up a 10 g/L aqueous solution:
▫ Weigh out 10 g of the substance.
▫ Put it in a vol. flask or grad. cylinder
▫ Add water up to the 1 L mark.
• If you mix 1 L water with 10 g solute, then the
total volume would be >1 L, and the
concentration would be <10 g/L.
Percent (parts per hundred)
• w/w = grams per hundred grams total
▫ (this applies to compounds in solid materials, too)
• v/v = mL per hundred mL solution
• w/v = g per hundred mL solution
• Example: to make 100 mL of 10% v/v methanol
solution, use 10 mL methanol and add water to a
total volume of 100 mL.
PPT, PPM, PPB
• Like percent, except more dilute
▫ parts per thousand
▫ parts per million
▫ parts per billion
• 1 ppm = 1 µg/g = 1 µg/mL = 1 mg/L
(1 g of water = 1 mL)
Molar (M) Solutions
• Definition: moles of solute per liter of solution
▫ 1 mole is 6x1023 molecules.
▫ The mass of 1 mole of a compound is the
molecular weight (MW) of that compound
• What you need to know:
▫ Volume desired
▫ Concentration desired
▫ Formula weight (FW)
(FW = MW + water of hydration, if any)
• A 1M solution is the FW of a substance in 1 liter
• Example 1:
▫ Make up 1 liter of 1M NaCl…
• Example 2:
▫ Make up 400 ml of 0.25M NaCl
• Example 3:
▫ Make up 1L of 5mM NaH2PO4
Normal (N) solutions
• Definition: The molarity of hydrogen ion
equivalents produced by a compound in solution.
(Usually applies to acids/bases.)
• What does this mean?
▫ For many chemicals the molarity and normality are
▫ 1M HCl is the same composition as 1N HCl
▫ Same for HNO3, HF, most organic acids
• Differences between molar and normal solutions
occur when you are working with molecules with
>1 exchangeable proton.
▫ Sulfate, phosphate, carbonate
▫ 18M sulfuric acid (H2SO4) is 36N
There are 2 H+ ions (protons) per molecule
Multiply the molarity of a solution by the number of
H+ to get normality
▫ 5M phosphoric acid (H3PO4) is 15N
Dilution from one concentration to
C1 = initial concentration
V1 = initial volume
C2 = desired final concentration
V2 = desired final volume
• You need 100 ml of 1M NaCl and you have a
stock solution of 5M NaCl
▫ What do you know?
C1 = 5M
C2 = 1M
V2 = 100ml
▫ Answer … solve for V1 = C2V2/C1
▫ = 1M x 100 ml/5M = 20 ml of the stock
Making very dilute solutions
• When making very dilute solutions (micromolar
concentrations, etc.) you may need to design a 2-
• You can’t weigh out <10 mg easily.
• So use more, make a concentrated stock
solution, and then dilute from the stock.
One other complication
• Sometimes a concentration is specified in terms
of one element in the solution:
▫ Make up 1 L of 10 ppm Zn solution
▫ That would be 10 mg Zn per liter.
▫ But elemental Zn is an insoluble metal!
• Use zinc sulfate (heptahydrate) – ZnSO4
▫ You’ll need to use more than 10 mg of zinc sulfate
to get the 10 mg of zinc that you want.
• Multiply amount of zinc needed by the ratio of
formula weight to atomic weight of Zn.
Special considerations for strong acids
• SAFETY: Whenever possible, add acid to water,
not water to acid.
▫ Important primarily for very concentrated acids
▫ Prevents splashing & over-heating of acid
• “Full-Strength” is not necessarily 100%.
▫ Ex.: 100% HCl is a gas! The most concentrated liquid
form is a 36% aqueous solution.
▫ So when diluting to make 10% HCl, you are starting
from 36%, not 100%.
▫ Acid concentration chart
• 1. Describe how you would make up
▫ 250 ml of 200 mM NaCl.
▫ 250 ml of a 0.05M glucose solution
▫ 10 ml of 10-5 M glucose solution starting from a
0.05 M stock solution
• 2. How many grams of acetic acid would you use
to make 10L of a 0.1 M solution of acetic acid?
• 3. What volume of 10 M acetic acid is required to
produce 1 liter of 0.5 M acetic acid?
• 4. Describe how to make a 2 N solution of
• 5. Describe how to prepare 500 ml of a 0.25M
solution of sodium hydroxide from a 1.0M stock
solution of sodium hydroxide.
More Practice Problems
For hints and answers to these problems go to:
1. Outline how you would prepare 250 ml of a 500 mM solution of potassium
chloride. For this problem only, and not for those that follow, give complete details
of what glassware you would use, and the mechanics of making the solution.
2. How would you prepare 100 ml of a 5 µM solution of ammonium nitrate (NH4NO3)?
3. Pete Paladin measures out 30 ml of 1M sucrose solution, and then adds water to
make a total volume of one liter. What is the molarity of the final solution?
4. Outline how you would prepare 1 liter of 10% lactic acid, from a bottle you would
probably find in the laboratory supply cabinet. What extra step should you take as a
5. Outline how you would prepare one liter of a solution of 5 ppm nickel. The chemical
available to you is nickel chloride (NiCl2·6H20).
6. What is the molarity of nickel in the 5 ppm solution prepared above?