To determine the stoichiometry of a chemical equation or reaction, we use molecular weight. It’s the sum of values of the atomic weight of molecules or atoms. Generally, we express molecular weight in terms of Daltons (Da) or atomic mass units (AMU) or are unitless. We define both molecular weight and atomic weight relative to the mass of the carbon isotope-12. It is assigned a value of 12 amu. As there is a mixture in the isotopes of carbon, the atomic weight due to this, the atomic weight of C is not accurately 12. 

Molecular Weight: A Short Description

We can understand molecular weight in terms of the molecule; it’s the sum of its atomic weight in it. It’s generally said as molecular mass. Different atoms are bound together to form a molecule with the help of some mutual interactions. 

For example, let’s take a water molecule to determine its molecular weight. We calculate H2O as (2*1.00797) +(1*15.9994) =18.0153. Here 15.9994 and 1.00797 are the atomic weight of oxygen (O) and hydrogen (O) respectively. We can determine molecular weight either by mass spectroscopy or even by chemical methods. We can determine any molecular weight like the molecular weight of oxygen and molecular weight of nitrogen.

Important Points Regarding Molecular Weight

• The approach to deciding relative molecular mass. Two French scientists planned this in 1819, Alexis-Thérèse Petit (1791–1820) and Pierre Louis Dulong (1785–1838). 
• They urged the quantity of heat needed for an atom to boost the temperature. This atom is of solid material by a given amount and must be independent of the kind of atom.
• The atomic weight ought to be proportional reciprocally to the material’s heat energy, that is, specific heat. 
• The law of Petit and Dulong verified a good approximation for several parts and many elements.
• It’s far from precise for several others. 
• This ended up being the least useful in telling the atomic weight of various elements of gas.
• In 1811, Avogadro finished those volumes of all gases that were equal at constant pressure and temperature, containing a constant range of molecules.
• But, Avogadro’s ideas had less influence on the work, and it was not till 1860 that some other Italian soul, Stanislao Cannizzaro came from 1826 to 1910.
• He noticed that Avogadro’s hypothesis could be used for determinative molecular weights and atomic weight.
• The methodology being used is density knowledge for determining the mass of a gas.  
• Experimentally, several millilitres of a volatile liquid are tempered during a closed flask with a little passage. 
• Then they heat the flask to a temperature higher than the boiling purpose of the liquid. 
• As soon as the liquid evaporates, its vapour replaces the air within the flask then it’s cooled.
• Thus, the vapour condenses as air re-enters the flask. 
• Then we calculate the mass of the vapour and employ conjunction with the best gas law to see the liquid mass. 
• This methodology works well for several gases.
• Although it can’t get in use for the material that gets decomposed on heating, like urea.

This is all about molecular weight. This tells the molecular weight of oxygen and molecular weight of nitrogen, and many others. 

How Can We Calculate The Molecular Weight?

The molecular weight calculation relies on the formula of a compound. That is not the only formula that solely includes the quantitative relation – these relations of kinds of atoms and not numbers. We multiply the amount of every style of an atom by its relative atomic mass or atomic weight, and then we add these to the weights of other adjacent atoms. 

Let’s say, for example, the formula of propane is C3H8. The subscripts indicate the amount of every style of the atom. Thus there are three carbon atoms and eight atoms of hydrogen in every molecule of propane. The relative atomic mass of hydrogen and carbon is also found in this table named periodic table.

Carbon atomic weight – 12.01

Hydrogen atomic weight – 1.01

We calculate molecular weight by

Molecular weight = number of atoms of carbon ✕ atomic weight of carbon + number of atoms of hydrogen* atomic weight of hydrogen

Thus, molecular weight = 3 ✕ 12.01 + 8 ✕ 1.01

Molecular weight= 36.03 + 8.08

Molecular weight= 44.11

Important Terms

• Isotopes

Two molecules in this type of atom and the number of atoms are identical. But the way they’re arranged in space is quite different. This results in different physical and chemical properties. 

• Avogadro’s number 

It’s denoted by N. the ‘n’ number of molecules in one mole is always equal to 6.0221415* 1023. It is named after Amedeo Avogadro. He is an Italian physicist. 

• Nuclides

This describes the matter that includes nuclei with certain values of atomic number and mass number. We denote atomic number with ‘Z’ and mass number with ‘A’. 

• Mass number

The mass number is the sum of several neutrons and the number of protons in an atom. We give the mass number a symbol of ‘A’.

Conclusion

This is all you need to know about molecular weight. It has a developed concept and follows specific models of chemistry. The molecular weight relatively depends upon the formula of the compound. Every compound attains some representative formula according to its inclusive elements, their atomic weight and mass. Hence based on these details the molecular mass of any element is determined. It attains a significant formula which helps it in mathematical calculation.