## What is Process Calculation?

A chemical or physical change in a substance or a group of substance that refers to any operation in series is called a basic chemical process and calculation of all these chemical processes is called process calculation.

In this context, we need to be familiar with basic chemical principles underlying the subject of discussion of the following terms:

• Atomic weight
• Molecular weight
• Basis of calculation.

### Atomic Weight

The mass of an atom is defined as its atomic weight.

### Molecular Weight

The sum of the atomic weights of the atoms using which a molecule of a compound is formed is defined as its molecular weight.

## Basis of Calculation

In solving the problems on material and energy balances of a given process, basis of calculations is very important. Therefore, we must assume a suitable basis on which further calculations are based before we start solving the problems on material and energy balances of a given process. Some suitable basis of calculation must be presented by assumption.

The basic basis of calculation must be some specific quantity or composition or flow rate of stream entering or leaving the process. Assume it as a basis of calculation if the flow rate is specified as a basis in the problem.

While assuming a new basis of calculation, it must be made sure that it must contain the maximum amount of information of the process. As, for example, if the stream amounts and flow rate are not specified, assume any amount of stream for which composition is known. The calculation is simplified on the basis of calculation of some amount or quantity of stream of which the composition is known.

## Concept of an Atom

An atom is the smallest particle of chemical element which can be part of a chemical reaction. It comprises of negatively charged electrons, no charged neutrons and positively charged protons. An atom has a nucleus at the centre. Nucleus has a mix of protons and neutrons and it is surrounded by electrons. Within the atom, these electrons are bound to the nucleus by the electromagnetic force.

The atom is said to be electrically neutral, if the number of electrons present in the atom is equal to the number of protons. The atom is said to be positively charged, if the number of electrons present in the atom is few relatively.

The atom is said to be negatively charged, if the number of electrons present in the atom is more relatively, then the atom is said to be negatively charged.

## Method of Expressing the Composition of Various Mixtures given

Different kinds of methods are used to express the composition of mixtures of solids, liquids and gases. Let us consider a system composed of two components, namely A and B. Similar methods are used for a system containing more than two components. The compositions can be expressed in weight per cent, volume per cent and mole per cent. Likewise, composition of solid systems and a liquid system is expressed in terms of weight per cent or mole per cent while the composition of gases is expressed in terms of volume per cent. The method of expressing the composition in ppm (i.e. one part in 106 parts) is generally found as a tool for specifying the contents of trace impurities in solids or liquids.

### Moles and mole fraction

Molecule is defined as a group of atoms of elements joined together to give rise to a specific structure. Atoms of the same element or the atoms of the different element may be formed using either.

A compound is a substance formed by the combination of two or more different elements. Sodium and chlorine are of two different elements combined together in sodium chloride. Therefore, NaCl is a compound.

Let us take the chemical reaction,

2Fe + 3H2O - Fe2O3 + 3H2

Here, iron is shown to be reacting with water to give rise to iron oxide and hydrogen. In this chemical reaction, iron and water are the reactants and iron oxide and hydrogen are the products. The molecular formula of the compound and elements present in this reaction helps us to calculate the amount of iron and water necessary to produce any desired amount of hydrogen.

### Equivalent weight

It is defined as the ratio of an atom’s atomic weight or a molecule’s molecular weight by its valency. This valency of an element or a compound depends on the number of hydrogen ions H+ accepted or the hydroxyl ion OH- donated for each of its atomic weight or molecular weight.

### Normality, molarity and molality

There are three ways to express the concentration of solution containing either a solid or a liquid solute, namely:

• Normality
• Molarity
• Molality.

### Concept of PPM (Parts Per Million)

When parts of one substance present in million parts of another substance, especially of solvent, it is commonly used as a unit of concentration. Concentration is the amount of any solute that is present in a known amount of solvent. It has the unit g/L or g/cc. For example, on considering the concentration of NaOH in water, when 5 g of this sodium hydroxide is dissolved in 1 litre of water, it gives the concentration of sodium hydroxide in water, that is, 5 g/L. When a large amount of solute substance is present in a solvent, it can be comfortably represented by the unit g/L. But, when the solute substance present in solvent is very small, the comfortless arises to represent the unit as g/L. To overcome this difficulty, ppm unit is used. This unit is most commonly used to measure the very small level of pollutants which are found in air. For example, Permissible Exposure Limit (PEL) of CO found in air is 50 ppm. It is also used as a measure of small level of pollutant present in drinking water. For example, permissible exposure limit of arsenic in drinking water is 1 ppm.

## Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses like Bachelors and Masters in Chemistry.

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