Molar Heat Capacity

While there are only 92 naturally occurring elements, there are innumerable compounds which are formed through their combination. Every compound is characterized by certain physical parameters and one of the most important ones is its molar heat capacity. The aim of writing this article is to explain what is molar heat capacity and explain how it’s calculated.

To comprehend what is molar heat capacity, one must first grasp the concept of what the term ‘Heat Capacity’ means. Heat capacity is the amount of heat that needs to be supplied to a substance of compound to raise its temperature by some degrees. Depending on the molecular structure of a substance and the kind of internal bonding, the heat capacity to raise a substance’s temperature, through a specific amount of degrees will vary. Specific heat capacity is the amount of heat (usually measured in joules), required to raise the temperature of a single gram of substance, through one degree Celsius or one degree Kelvin.

What is Molar Heat Capacity?
You need to know what is a ‘Mole’ of a substance to fully understand what is its molar heat capacity. The molecular weight of the substance, expressed in grams is known as a single mole of that substance. A mole of a substance, is known to contain 6.023 x 1023 molecules. For example, the molar mass of water is approximately 18 gm/mol.

The molar heat capacity of a substance is the amount of heat required to raise the temperature of one mole of a substance or compound through unit degree Celsius. Since the molecular weight and structure of each compound varies, every substance has a unique molar heat capacity value.

What are the Units of Molar Heat Capacity?
Every physical quantity, including the molar heat capacity needs to be specified with the proper units. The units used for specifying the molar heat capacity are ‘Joule / mol K’ or ‘Joule Per Mole Kelvin’. Let us now take a look at the molar heat capacity formula.

Molar Heat Capacity Equation
It is theoretically possible that the molar heat capacity of a substance, can be calculated from its specific heat capacity. Here is the formula which can be used to deduce the molar heat capacity from specific heat capacity.

Molar Heat Capacity (Joules / Mol K ) = [Specific Heat Capacity (Joules / gm K) x Molar Mass (gm/Mol)]

The above formula simplifies molar heat capacity calculation substantially, if you have the value of specific heat capacity of the substance and the knowledge of its molar mass.

How is Molar Heat Capacity Calculated?

To use the above formula for calculation of molar heat capacity of any substance, you need to know how to calculate molar mass and a reference chart that has the specific heat capacity of that substance. Let me demonstrate the calculation of molar heat capacity of liquid water.

The molar mass of water is approximately 18 gm/mol and the specific heat capacity of water is 4.1813 Joule / gm K (at 25 degree Celsius). So the molar heat capacity of water will be:

Molar Heat Capacity of Water = (4.1813 Joule / gm K) x (18 gm / mol) = 75.2634 Joule / Mol K

Knowing the molar heat capacity of any element is an experiment which you will surely perform as a part of your first chemistry lab course. It is an important step in what is known as the characterization of any compound. Hope this article has left no doubt in your mind about how to calculate molar heat capacity of any compound. You could either determine it experimentally or theoretically from the knowledge of specific heat capacity.

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