Chemical Solutions

Several basic skills are required to be successful in the biology laboratory. One of them is the ability to make chemical solutions of known concentration that will be used for experimental, therapeutic and diagnostic purposes.

Making a solution of known concentration

Concentrations (amount per unit volume) can be expressed in several ways.

Weight per unit volume: One mixes a weighed amount of solute with a known volume of solvent (usually water unless otherwise indicated) to make a solution of a known concentration.

For instance, to make a 4 g/liter NaCl solution one would weigh on a balance 4 g of NaCl and dissolve it in 1 liter (same as 1000 ml) of water.

In the laboratory, the source of water is very important and is almost never tap water, which contains a variety of ions and other chemical contaminants. Use water that has been deioinized. All modern labs are equipped with filtration or deionization units to supply such water.

If a different volume is needed, the amount of solute is determined by a simple proportion.

For example for 250 ml at the same concentration as above, the amount of solute needed is calculated by:

x g / 250 ml= 4 g / 1000ml

x= 1 g

Thus , weigh 1 g of NaCl and dissolve in 250 ml of water.

Percent Solutions: Since these are expressed as percentages, they are parts of solute to 100 parts of solution. This can be expressed as w / w (weight to weight) or w / v. Conventionally, most biological applications assume w / v unless otherwise specified.

For example, 5% NaCl solution (w / v) is 5 g NaCl dissolved in water to give a final solution volume of 100 ml. If the recipe calls for 5% (w / w) NaCl solution, one mixes 5 g NaCl with water to a final of 100 g of solution. (Since the density of water is 1 g/ ml, would the volume of solution differ in these two percentage solutions?)

Molar solutions: A mole is the atomic or molecular weight of an Avagadro's number (6 x 10 23) of substance in grams. A molar solution is the number of moles of solute per liter of solution.

For example, 1 M (molar) is equal to one mole of a substance in 1 liter of solution.

The atomic or molecular weight (MW) of a substance can be found by looking at a periodic table. In the lab, one typically finds this value on the label of the substance's container.

For example, the molecular weight of NaCl is (Na) 23g / mole + (Cl) 35.4 g / mole =58.4 g/mol (MW)

To make a 2M solution, weigh (2 moles X 58.4 g / mole =) 116.8 g NaCl and dissolve in water to a final volume of 1 liter.

A useful formula is

[C] = mass of solute / MW of solute
..............volume of solution

If mass is in grams and volume is in liters, then [C] is the molar concentration.

Commonly, solutions of less than 1M are used in the lab such as millimolar or micromolar solutions. Note that 1 M = 1000 mM (i.e. 0.1M = 100 mM;
0.01 M = 10mM; 0.001 M = 1 mM)

1 mM= 1000 mM (What is another way of expressing 0.01 mM?)

Another useful formula is:

C1V1=C2V2

Suppose one wants to make 2 ml of a solution with a final concentration of 10 mM Tris pH7.5 and 0.5 M NaCl. Available are two concentrated stock solutions of 1 M Tris pH 7.5 and 5 M NaCl. How much volume from these stocks and how much water will one need ?

Tris
C1= 1 M
C2= 0.01 M
V2= 2 ml

Solve for V1
V1= 0.02 ml

NaCl
C1= 5 M
C2= 0.5 M
V2= 2 ml

Solve for V1

V1= 0.2 ml

One would mix together 0.02 ml of 1 M Tris pH 7.5, 0.2 ml of 5 M NaCl and 1.78 ml deionized water.

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Name:_______________________________________________________________________

I. Chemical Solution Problems

Describe how to make the following solutions. Show your calculations.

1. 5 ml of a 2 mg/ml CuSO4 solution.

2. 100 ml of a 1% glucose solution

3. 150 ml of a 1 M NaCl solution

4. 100 ml of a 5 mM NaCl solution using a 1M NaCl stock (not the crystal).

II. pH Problems

Answer each question and please show your work. Don't forget units.

Note that [ ] is shorthand for molar concentration e.g.[H+] = the molar concentration of hydrogen ion; [OH-] = the concentration of hydroxide ion.

1.a. What is the mathematical definition of pH?

b. At what pH are the hydrogen and hydroxide ion concentrations equal?

c. What are the concentrations of the ions in part b?

2. a. If a solution is pH 5, what is the [H+]?

b. For the same solution, what is the [OH-]?

3. a. Is pH 13 basic (alkali) or acidic?

b. What is the [H+]?

c. Is this greater or less than the [OH-]?

4. When a solution, which is initially neutral, is adjusted to pH 4, how much does the [H+] change?

a. 3x greater

b. 3x less

c. 1000x greater

d. 1000x less

e. none of the above