LABORATORY CALCULATIONS

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Molecular weight is the sum of the atomic weights of all the atoms in a molecule of substance. (One mole is the same as one molecular weight.)

Example: What is the molecular weight of MgCl2?

Substances that have water of hydration should have this included in the molecular weight.

Example: What is the molecular weight of MnSO4 · 4H2O?

Molarity is concentration of a solution that indicates the number of moles of solute in one liter of solution.

Example: Prepare a 2 M NaCl solution.

Add 116.8 g NaCl to water and dilute to a final volume of 1 liter.

If a smaller volume is needed, reduce the amounts of solute and solvent by the same fraction.

Example: Prepare a 250 ml solution of 2 M NaCl.

0.25 x 2(m. w. NaCl) = 14.6 g

Add salt to water and dilute to a final volume of 250 ml.

Many solutions can be made 10 x concentrated and diluted as needed. The following can be used to prepare these dilute solutions:

M original/M final = V final solution/V original solution

Example: What is the amount of 10 M NaCl needed to make one liter of a 0.1 M solution?

Percent is the concentration of a solution that indicates the number of parts solute in 100 parts of solution in volume or weight units.

Assumption: At 20°C, 1 g water = 1 ml water.
For practical purposes, other solvents used in the laboratory have densities that are
approximately the same as water.

a. SOLUTIONS PREPARED AS PERCENT BY WEIGHT
(Most solutions prepared in the laboratory are by percent weight when the solute is solid and the solvent is water.)

1. Concentrated solutions (5% or more).

Example: Number of grams of sucrose in a 20% solution?

By definition, this means 20 g of sucrose are in 100 g of solution. Therefore, 80 g of water would be needed to prepare the solution. (The volume of this solution would be 92.5 ml.)

2. Dilute solutions by weight (less than 5%).

Assumption: Solutes do not significantly increase the volume of the solution. Therefore, the volume of the solvent is approximately equal to the volume of the solution.

Example: Number of grams of NaCl in 100 ml of a 0.2% aqueous solution?

By definition, this means 0.2 g of salt in 100 g of solution, or 0.2 g dissolved in 100 ml of water.

b. SOLUTIONS PREPARED AS PERCENT BY VOLUME
(Usually used when a liquid is being diluted by water.)

Volume original x percent original = Volume final x percent final

Example: Prepare 100 ml of a 70% ethanol solution using 95% ethanol.

A simple way to prepare this solution in the laboratory without calculation is the following approximate rule of thumb:

In a graduated cylinder, measure 70 ml of the ethanol and dilute with water to 95 ml mark.

pH is a measure of the hydrogen ion concentration expressed as a negative logarithm.

Solute is material that is dissolved in the solvent. This usually is dry and measured in grams.

Solvent is material that dissolves the solute. Usually, this is a fluid; the most common being water, which is measured in liters.

Specific gravity is the ratio of the density of a substance to the density of water.

s.g. = density of reagent (g/1)/density of water (g/1)

The specific gravity of a substance numerically equals its density because the density of water is one.

Example: Commercial hydrochloric acid is 37% by weight and has a specific gravity of 1.2. What is the
molarity of the acid?

By definition,

 

GUIDES AND REFERENCES

The following references are very useful:

  1. Difco Manual of Dehydrated Culture Media and Reagents. Detroit, MI: Difco Laboratory.
  2. Handbook of Chemistry, Lange’s. New York, NY: McGraw-Hill.
  3. Handbook of Chemistry and Physics. Boca Raton, FL: CRC Publishers.
  4. Merck Index. Rahway, NJ: Merck and Company.

 

All the above are periodically revised.