Term

 Definitions

Solution

The term solution used in chemistry, biochemistry, biology, molecular biology, medicine, and other professional fields refers to a homogenous mixture of two or more substances. Usually, when performing experiments in the life sciences, the term solution refers to aqueous solutions. Here the solvent is water.

 Solute

The solute is the minor species of the mixture dissolved in or mixed with the solvent's major species.

Solvent

The term solvent refers to the major species of a solution.

Concentration

A substrate or sample concentration refers to how much solute is contained in each volume or mass of solution or solvent.

Atomic Weight (AW)

The atomic weight of an element is the number of grams containing Avogadro’s number of atoms. The molecular weight of a compound is the sum of atomic weights.

Molecular Mass

The molar mass of a substance is 1 mol (the substance's SI unit).

The numerical value is the molecules' average molecular mass in the compound multiplied by Avogadro's constant (NA  = 6.02214129(27)×10²³ atoms or molecule mol⁻¹).

However, many analysts commonly use the atomic mass unit (u or Da). For example, the molecular mass of water is close to m(H₂O) ≈ 18 u ≈ 3×10⁻²⁶ kg.

The molecular mass is sometimes called molecular weight, which is incorrect because mass and weight are different. The existence of isotopes complicates the picture further. The summing up of the molecular masses of individual isotopic abundances found in the extended periodic table of all the atoms in any molecule allows calculating molecular masses. For calculating working estimates for most molecular biology experiments calculating the average mass is sufficient.However, accurate masses, including the isotope mass pattern, are needed to verify the predicted mass experimentally.

Molecular weight (MW)

The molecular weight (mol. wt. or Mw) of a molecule is the sum of the atomic weights of all the atoms in a molecule sometimes called the formula weight. It is the number of grams containing Avogadro’s number of molecules.

Avogadro’s number

6.02214084(18) x 10²³ mol^-1.

This is the latest measured accurate number! (http://arxiv.org/abs/1010.2317).

The Avogadro constant or number links the atomic and the macroscopic properties of matter, however, the accurate measurement of this number is not trivial. Accurate determinations of Avogadro’s number require measurements of a single quantity on both the atomic and macroscopic scales using the same unit of measurement. The best estimate of the value of a Faraday, according to the National Institute of Standards and Technology (NIST), is 96,485.3383 coulombs per mole of electrons. The best estimate of the charge on an electron based on modern experiments is 1.60217653 x 10^-19 coulombs per electron. If the charge on a mole of electrons is divided by the charge on a single electron a value of Avogadro’s number of 6.02214154 x 10²³ particles per mole is obtained. This is from an older measurement.

6.0221367(36) x 10²³ mol^-1 is the result of another older measurement.

For how to determine Avogadro’s number review this article. (http://www.scientificamerican.com/article/how-was-avogadros-number/)

Amedeo Avogadro (https://www.britannica.com/biography/Amedeo-Avogadro)

Mole (mol)

A mole or mol refers to Avogadro’s number of molecules.

Molarity (M)

Molarity is the number of moles of a substance per liter solution. Molarity changes with temperature because the volume of a solution usually increases when heated.

Liter (L)

A liter is the volume of a cubic that is 10 cm on each edge. 10 cm = 0.1 m; 1 L = (0.1 m)³ = 10^-3 m³. Chemical concentrations are usual expressed in moles per liter. Therefore “[H⁺]” means “the concentration of H⁺.”

Molality (m)

Molality is the designation of concentration expressing the number of moles of substance per kilogram of solvent (not the total solution). Molarity is independent of temperature.

Percent Concentration

The percentage of a component or solute in a mixture or solution is usually expressed as

weight percent (wt %):

Weight percent = (mass of solute)/(mass of total solution or mixture) x 100.

For example, 95 wt % of ethanol (CH₃CH₂OH) contains 95 g per 100 g of total solution. The rest is water.

Volume percent (vol %), on the other hand, is defined as

Volume percent = (volume of solute) / (volume of total solution) x 100

Units of weight or volume should always be expressed to avoid a mix-up.

Reference

Quantitative Chemical Analysis. Fifth Edition. Daniel C. Harris. W. H. Freeman and Company New York. 1999. ISBN 0-7167-2881-8.