[Chemistry Class Notes] on Concept of Molar Mass Pdf for Exam

The molar mass of a chemical compound in chemistry is defined as the mass of a sample divided by the amount of substance in the sample that is measured in moles. 

The molecular weight is usually used as a synonym of molar mass especially for molecular compounds, although it is very different as is known as molecular mass.

What is Molar Mass?

Molar mass is a smallest unit of a compound with one twelfth of the mass of one carbon – 12 atoms. When we know the number of moles needed, the concept of molar mass can be used to calculate how many grams of substance are required. The molar mass otherwise known as molecular weight is the sum of the total mass in grams of all the atoms that make up a mole of a molecule. The unit required to measure is grams per mole.

Formula of Mole

1. Mass of 1 mole of atom = Ar in grams

2. Number of moles of atom = mass of the element in grams/ relative atomic mass, Ar

3. Mass of 1 mole of molecules = Mr in grams

4. Number of moles of molecules = mass of the substance in grams / relative molecular mass, Mr

5. Mass of substance the contains 1 mole of particle = molar mass

6. Percentage yield = actual mass of product obtained / theoretical mass of product obtainable.

Chemical Computation with Mole and Avogadro’s Number

A mole is the SI measure of the quantity of a ‘chemical entity’ such as electrons, protons, or atoms. 1 mole contains 6.022 × 1023  elementary entities of the substance. Avogadro’s number is fundamental for understanding both the makeup of molecules and their combinations and interactions. For instance, since one atom of oxygen combines two atoms of hydrogen to create one molecule of water (H2O), one mole of oxygen (6.022 × 1023  of O atoms) combines with two moles of hydrogen (2 × 6.022 × 1023  of H atoms) to make one mole of H2O. 

Another Avogadro’s number property is that the mass of one mole of a substance is equal to the molecular weight of the substance. For instance, the mean molecular weight of water is 18.015 atomic mass units. Therefore, one mole of water weighs 18.015 grams. It simplifies many chemical computation.

 

Steps to Find Molar Mass for Compounds

Compounds are those substances that are made up of one or more elements. Examples of some common compounds include glucose, salt, acetic acid, and sodium bicarbonate. 

Sodium chloride compound is made up of two elements i.e., sodium and chloride. We will use sodium chloride as one of the examples to calculate the molar mass for the compounds 

Step 1: Find the atomic mass of individual element in the periodic table

We have to first find the atomic mass for each element. The element sodium has the atomic mass of 22.98976g/mol. Chlorine has an atomic mass of 35.453 g/mol.

Step 2: Count atoms for each element

As there are no subscripts for compound sodium chloride, it means it has only one sodium and one chlorine atom for the compound. 

Step 3: find the molar mass

Now, we are able to find the molar mass as we know the number of atoms for each element. Here we calculate first the mass of the sodium atoms that is 22.98976 g/mol. we will repeat the same for mass of chlorine atoms that is 35.453 g/mol, now we have to add these two masses together to find the total mass of molecules of sodium chloride. The total sodium chloride molecules are 58.44276 g/mol which we can round up to 58.44 g/mol.

Na = 1 × (22.98976 g/mol) = 22.98976 g/mol

Cl = 1 × (35.453 g/mol) = 35.453 g/mol

Molar mass = 22.98976 + 35.453 g/mol

Molar mass = 58.44276 or 58.44 g/mol

Solved Examples of Molar Mass

1. When you have 1.25 grams of a molecule with a molecular weight of 134.1 g/mol, find out how many moles of that molecule you have?

Solution: 1.25 g × 1 mole / 134.1 g = 0.0093 grams.

2. Calculate the mass of 6.022 × 1023 molecules of NH4Cl?

Solution: Molar mass in grams of NH4Cl = 14 + 4 +35.5 = 53.5 g

No. of moles of NH4Cl = 6.022 × 1023  / 6.022 × 1023  = 1 mole.

Now, mass of NH4Cl = number of moles × molar mass

                                         = 1 × 53.5 g

      = 53.5 g

3. Calculate the number of methane molecules and the number of carbon and hydrogen atoms in 25 g of methane?

Solution: Molar mass of methane = 16

Number of methane molecules = 25/16 × 6.022 × 1023  

    = 9.411 × 1023  

Number of carbon molecules = 1 × 9.411 × 1023  

  = 9.411 × 1023  

Number of hydrogen molecules = 4 × 9.411 × 1023  

      = 3.74 × 1023