Masses, moles and concentrations

SCIENCE IN ACTION

By MICHAEL JOHN UGLO
IN this eighth lecture, we will look at the great ideas on Masses, Moles and Concentrations.
In any chemical reaction, it is appropriate to quantify what is involved in the chemical reaction so, it can be mathematically calibrated.
It is very important to say it with figures and numbers when talking about the quantity of chemicals used to come up with specific products. These are any detailed information about any form of matter and in certain proportions to derive any product. For instance, people working in a laboratory have different kinds of matter labeled as chemicals that they want to use to combine in chemical reactions to come up with new products in an experiment.
In this situation the masses of the matter that are worked with are to be known. Matter is specifically reduced to elements, compounds, ions and plasmas. Their masses and weights have to be known to determine correctly how much is used or required to come up with certain quantities of products required. In this instance, mass is measured in grams unlike in physics whereby mass is measured in newtons. For instance, atomic mass of atoms and molecules are measured in grams. Also such quantity as molarity is measured in grams per mole.
The reference point that is used is the isotope carbon-12. The same element carries different mass numbers with the same atomic number and these are isotopes of the same element. There are other isotopes of the element carbon and one such is carbon-13 and the other is Carbon-14. Carbon-14 is used in carbon-dating.
For measurements, the international metric system of gram is always used in this calculation. That is, an atom of Carbon-12 weighs 12 Units and that is 12 grams of carbon-12 is equal to this standard unit.
Practically, 12 grams of carbon-12 placed on a tea spoon is equal to 6.23 x 1023 carbon-12 atoms on the spoon. The 12 grams contains like 6.023 x 1023 number of particles of carbon-12 atoms. This number is called the Avogadro’s number or Avogadro’s constant. It refers to particles including atoms, molecules or even ions and plasma of this size.
The atomic weights and the molecular weights are compared to carbon-12 for the number of moles of that element. For instance, magnesium with an atomic mass of 24 is two moles with the standard unit with carbon-12. That is 24/12 gives 2, therefore it is two moles.

Moles are hence, the Avogadro’s constant of carbon-12 as the standard unit that specify the particle’s quantity whether it be atoms or molecules or any species like ions and plasma of an element or a compound. One mole of gas is equal to its relative molecular mass given in grams (not in newtons as in physics) just like a mole of a pure substance is its mass measured in grams that is numerically equal to its molecular mass in atomic mass unit.
The molar volume of a gas is moles of any gases which has a volume of 24dm3 or 24,000cm3 or 22.4L at rtp (room temperature and pressure). This is defined in the ideal gas law also known as general gas equation. This law ties the amount of gas in moles to its temperature, pressure and volume.
The law is defined in this equation as PV=nRT whereby P, V and T are respectively pressure, volume and temperature. The n is the amount of substance and R is the ideal gas constant at the value 8.314J/mol.K. It (R) is the same for all gases. The ideal gas law thus combines all the three primary laws, the Charles’ Law, Boyle’s Law and the Avogadro’s law.
Concentration is equals to the moles divided by the volume (Conc = Moles/Vol). The definition of molarity as the concentration of a solution in terms of the number of moles of the solute in 1dm3 as 1 liter of the solution is referred to the above formula for the calculation of concentration given in the units mol/L or just M.

The molarity of a solution can be defined from the following formula and that is M = n/v. This is defined as M = Molarity, n = moles of solute and v = liters of solution. Thus, molarity of a solution can be obtained by dividing the moles of solute by the volume of solution given in liters. The volume is taken as liters of solution and not as liters of solvent. The symbol for molarity is M and M stands for molar.
Generally, concentration is the presence of a particular species of particles divided by the total volume of the solution. The molar concentration is the concentration of the amount of that specific substance.
A concentrated solution could not contain the maximum solutes. Further solutes can be added in a concentrated solution while in a saturated solution it contains the maximum solute in a solution. When further solutes are added, it cannot be dissolved any further but rather falls and collects at the bottom of the solution.
My Prayer for PNG today is: “Before the sun burned bright and rivers flowed, I called you each by name, to share my own…”
Next week: Energy and reaction rates

• Michael Uglo is a science textbook author and lecturer in avionics, auto-piloting and aircraft engineering. Please send comments to [email protected]