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What is molar mass?

Molar mass

In the field of chemistry, it is important to know the molar mass of the elements that are going to be worked with in experiments in order to have a measure of the exact weight of the matter present in the body or bodies that are going to interact in the research. The molar mass is linked to the weight of entities such as atoms, molecules, ions, etc., of each element . All elements have a different molar mass because their atoms have different numbers of protons, electrons, and neutrons, which affects their atomic masses. To calculate the molar mass it is necessary to make use of the periodic table of elements.

Molar mass

  • Symbol: M
  • Unit: kg / mol and g / mol

What is molar mass?

Molar mass is the relationship that exists between the mass of a substance and the number of particles that compose it . This is expressed in kilograms per mole but it is more common to see it presented in grams per mole (g / mole). Its calculation is of great importance in the field of chemistry for the development of experiments with elements, compounds or mixtures of different substances.

  • What is molar mass used for
  • Molar mass symbol
  • Units of molar mass
  • How to calculate molar mass
  • In the elements
  • In compounds
  • In mixtures
  • Examples

What is molar mass used for

Knowing the molar mass allows us to: calculate the specific amount of a substance; analyze the results of the experiments; calculate percent by mass and understand precise molar masses of complex molecules without directly experimenting with them.

Molar mass symbol

The symbol M is the molar mass . It is important not to confuse it with the symbol for molecular mass which is m.

Units of molar mass

Its unit of measurement according to the international system is Kilogram per mole (kg / mol or kg x mol -1 ) , but it is almost always expressed in grams per mole (g / mol)

How to calculate molar mass

The molar mass can be calculated from the atomic masses expressed in the periodic table of elements . It is almost never calculated directly.

It is important to mention that the constant value of a mole is 6.022 × 10 23 this constant is called Avogadro’s number or constant.

Next we will present how it is calculated in elements0, compounds and mixtures.

In the elements

The molar mass of an element is given by the atomic weight of each element multiplied by the molar mass constant (6.022 x 10 23 ). The numerical value of the molecular mass and the molar mass are equal. The difference is that the value of the atomic mass is expressed in atomic mass units and the molecular mass refers to a single molecule ; the molar mass corresponds to one mole of molecules.

For example, to calculate the molar mass of hydrogen we must first look up its relative atomic mass on the periodic table. This is 1.007. The molecular mass of hydrogen is equal to the relative atomic mass times the number of grams / mole, in this case it is two because hydrogen is made up of two molecules. The formula would be:

  • M of H = Atomic mass of hydrogen g / mol x 2
  • M of H = 1.007g / mol x 2
  • M of H = 2.014 g / mol (this is the mass of hydrogen in one gram / mol)

In compounds

The molar mass of a compound is calculated by the sum of the standard atomic weights of the atoms that make up the compound multiplied by the molar mass constant (6.002 × 10 23 ).

For example, to calculate the mole grams of hydrochloric acid it is necessary to know the values ​​of the elements that make up this compound in the periodic table. Hydrochloric acid is made up of a hydrogen atom and a chlorine atom, thus symbolized HCL. The relative atomic mass of hydrogen is 1.007 that of chlorine is 35.453 and its weights are 1.007 g / mol and 35.453 g / mol. The formula would be:

  • M HCl = M H (1.007g / mol x 1) + M CL (35.453 g / mol x 1)
  • M HCl = 1.007 g / mol + 35.453 g / mol
  • M of HCL = 36.460 g / mol (This is the mass of the hydrochloric acid)

In mixtures

The molar mass of a mixture is calculated using the molar fractions (Xi) of the compounds and their molar masses (Mi) or from the mass fraction (wi) of the compounds .

The formula to calculate the molar mass of the mixtures is MM = 1 / ∑ (Xi MMi) 

For example, if we want to calculate the molar mass of a mixture of 181.463 g composed of 44.06844 g of ethanol, 18.0153 g of water and 119.38 g of chloroform.

The first step is to calculate the mass fraction of each of the substances.

The mass fraction of substances is calculated by dividing the mass of the substance by the mass of the mixture.

The formula would be:

  • Mass fraction of ethanol = 44.06844g / 181.463g = 0.2428
  • Mass fraction of water = 18.0153 g / 181.463 g = 0.0992
  • Chloroform mass fraction = 119.38 g / 181.463 g = 0.6578

The second step is to apply the average molar mass formula using the formula

MM = 1 / ∑ (Xi MMi)

Where Xi = mass fraction of substances and MMi = mass of substances

  • MM = 1 / ((0.2428 / 44.6844 g / mol) + (0.0992 / 18.0153 g / mol) + (0.6578 / 119.38))
  • MM = 1 / 0.0165 g = 60.606 g / mol

Examples

Next we will present the molar mass of some elements, gases and mixtures.

Example 1

The chemical formula of propane is C 3 H 8:

  • MC = 12.011 g / mol x 3 = 36.033
  • MH = 1.007 x 8 = 8.056
  • MC 3 H 8 = 36.003 g / mol + 8.056 g / mol
  • MC 3 H 8 = 44.089 g / mol

Example 2

The chemical formula of sulfur is S 16:

  • MS = 32.065 g / molx 16
  • MS = 513.04 g / mol

Example 3

The chemical formula of sulfuric acid is H 2 SO 4:

  • MH 2 = 1.007g / mol x 2 = 2.014 g / mol
  • MS = 32.066 g / mol x 1 = 32.066 g / mol
  • MO 4 = 15,999 g / mol x 4 = 63,997
  • M of H 2 SO 4 = 2.014 g / mol +32.066 g / mol + 63.997 g / mol = 98.077 g / mol

Example 4

The chemical formula of calcium is Ca 20:

  • M Ca = 40.078 g / mol x 20 = 801.560 g / mol

Example 5

The chemical formula of magnesium is Mg 12:

  • M Mg = 24.305 g / mol x 12 = 291.659 g / mol

Example 6

The chemical formula of glucose is C 6 H 12 O 6:

  • MC = 12.011 g / mol x 6 = 72.066 g / mol
  • MH = 1.007 g / mol x 12 = 12.084 g / mol
  • MO = 15,999 g / mol x 6 = 95,996g / mol
  • MC 6 H 12 O 6 = 72.066 g / mol + 12.084 g / mol + 95.996g / mol = 180.146

Example 7

The chemical formula of copper Cu 29:

  • M Cu = 63.546 g / mol x 29 = 1842.834 g / mol

Example 8

The chemical formula of aluminum is Al 13:

  • Al = 67.059 g / mol x 13 = 390.838 g / mol

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