in practical and theoretical chemistry, and there are two practical importance of concepts such as molecular (it is often replaced by the concept of molecular weight is not correct), and the molar mass.Both of these values are dependent on the composition of a simple or complex substance.
How to determine the molar mass or molecular?Both of these physical quantities is impossible (or nearly impossible) to find a direct measurement of, for example, weighing the matter in the balance.They are calculated from the chemical formula of the compound and the atomic masses of all the elements.These values are numerically equal, but differ in dimension.The molecular weight is expressed in atomic mass units, which are notional value and are marked.e. m., as well as another name - "daltons".Units expressed in molar mass in g / mol.
molecular weights of simple substances whose molecules consist of a single atom, equal to their atomic masses, which are specified in the periodic table.For example, to:
- sodium (Na) - 22,99 as well.eat.;
- iron (Fe) - 55.85 a.eat.;
- sulfur (S) - 32,064 a.eat.;
- argon (Ar) - 39,948 a.eat.;
- potassium (K) - 39,102 a.e. m.
Also, the molecular weights of simple substances whose molecules consist of several atoms of a chemical element is calculated as the product of the atomic mass of the element in the number of atoms in the molecule.For example, to:
- oxygen (O2) - 16 • 2 = 32 a.eat.;
- nitrogen (N2) - 2 • 14 = 28 a.eat.;
- chlorine (Cl2) - 35 • 2 = 70 a.eat.;
- ozone (O3) - 16 • 3 = 48 a.e. m.
Calculate the molecular weight of the substance, adding the product of the atomic weight by the number of atoms in a molecule which is part of each element.For example, for:
- hydrochloric acid (HCl) - 35 + 2 = 37 a.eat.;
- Carbon Monoxide (CO) - 12 + 16 = 28 a.eat.;
- carbon dioxide (CO2) - 12 + 16 = 44 • 2 a.e. m.
But how to find molar mass of the substance?
It's easy to do, since it is the weight of the unit amount of a particular substance, expressed in moles.That is, if the calculated molecular weight of each substance is multiplied by a constant value equal to 1 g / mol, it will turn its molar mass.For example, how to find the molar mass of the carbon dioxide (CO2)?It should be (12 + 16 • 2) • 1 g / mole = 44 g / mol, i.e. MSO2 = 44 g / mol.For simple substances, molecules that contains only one atom of the element, this indicator expressed in g / mol is numerically equal to the atomic mass of the element.For example, sulfur MS = 32,064 g / mol.How to find the molar mass of a simple substance, whose molecule is composed of a few atoms, we can consider the example of oxygen: MO2 = 2 • 16 = 32 g / mol.
There were examples of specific simple or complex substances.But whether and how to find the molar mass of the product, which consists of several components?As the molecular, molar mass of multi-component mixture is an additive quantity.It is the sum of products of molar mass component of its share in the mix: M = ΣMi • Xi, that is, can be calculated as the average molecular and average molar mass.
For example air, which includes approximately 75.5% nitrogen, 23.15% oxygen, 1.29% argon, 0.046% carbon dioxide (the remaining impurities are contained in smaller amounts, can be ignored): Mvozduha = 28 + 0.755 • 32 • 40 • 0,2315 + 0.129 + 44 • 0,00046 = 29,08424 g / mole ≈ 29 g / mol.
Find the molar mass of the substance, if the accuracy of the atomic weights specified in the periodic table, different?For some items it contains up to a tenth, to others in hundredths, thousandths to third, and for such as radon - to as much as for the manganese to the ten-thousandths.
When calculating molar mass does not make sense to conduct calculations with greater accuracy than to the nearest tenth, as they have a practical use, when the purity of the chemicals themselves or reagents will make a big error.All these calculations are approximate.But wherever chemists required great precision, using certain procedures shall be amended accordingly: Set the titer of the solution, made calibration standard samples, and more.
