Fundamenta
Fundamental Concepts of General Chemistry
Molecular weight or Molecular Mass
Molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained
by multiplying the atomic mass of each element by the number of its atoms and adding them
together. For example, molecular mass of methane, whic
whichh contains one carbon atom and four
hydrogen atoms, can be obtained as follows:
Molecular mass of methane, (CH4) = (12.011 u) + 4 (1.008 u)
= 16.043 u
Similarly, molecular mass of water (H 2O) = 2 × atomic mass of hydrogen + 1 × atomic mass of
oxygen
= 2 (1.008 u) + 16.00 u
= 18.02 u
Formula Mass
Some substances, such as sodium chloride, do not contain discrete molecules as their constituent
units. In such compounds, positive (sodium ion) and negative (chloride ion) entities are arranged
in a three-dimensional
dimensional structure, as shown in Fig. 1.
Fig. 1 Packing of Na+ and Cl– ions in sodium chloride
,It may be noted that in sodium chloride, one Na+ ion is surrounded by six Cl– ion and vice-versa.
The formula, such as NaCl, is used to calculate the formula mass instead of molecular mass as in
the solid state sodium chloride does not exist as a single entity.
Thus, the formula mass of sodium chloride is atomic mass of sodium + atomic mass of chlorine
= 23.0 u + 35.5 u = 58.5 u
Stoichiometry and Stoichiometric Calculations
The word ‘stoichiometry’ is derived from two Greek words — stoicheion (meaning, element)
and metron (meaning, measure). Stoichiometry, thus, deals with the calculation of masses
(sometimes volumes also) of the reactants and the products involved in a chemical reaction.
Stoichiometry is an important concept in chemistry that helps us use balanced chemical
equations to calculate amounts of reactants and products. Before understanding how to calculate
the amounts of reactants required or the products produced in a chemical reaction, let us study
what information is available from the balanced chemical equation of a given reaction. Let us
consider the combustion of methane. A balanced equation for this reaction is as given below:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
Here, methane and dioxygen are called reactants and carbon dioxide and water are called
products. Note that all the reactants and the products are gases in the above reaction and this has
been indicated by letter (g) in the brackets next to its formula. Similarly, in case of solids and
liquids, (s) and (l) are written respectively.
The coefficients 2 for O2 and H2O are called, stoichiometric coefficients. Similarly the
coefficient for CH4 and CO2 is one in each case. They represent the number of molecules (and
moles as well) taking part in the reaction or formed in the reaction.
Limiting Reagent
Many a time, reactions are carried out with the amounts of reactants that are different than the
amounts as required by a balanced chemical reaction. In such situations, one reactant is in more
amount than the amount required by balanced chemical reaction. The reactant which is present in
the least amount gets consumed after sometime and after that further reaction does not take place
,whatever be the amount of the other reactant. Hence, the reactant, which gets consumed first,
limits the amount of product formed and is, therefore, called the limiting reagent.
Reactions in Solutions
A majority of reactions in the laboratories are carried out in solutions. Therefore, it is important
to understand as how the amount of substance is expressed when it is present in the solution. The
concentration of a solution or the amount of substance present in its given volume can be
expressed
ed in any of the following ways
ways:
1. Mass per cent or weight per cent (w/w %)
2. Mole fraction
3. Molarity
4. Molality
,
Fundamental Concepts of General Chemistry
Molecular weight or Molecular Mass
Molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained
by multiplying the atomic mass of each element by the number of its atoms and adding them
together. For example, molecular mass of methane, whic
whichh contains one carbon atom and four
hydrogen atoms, can be obtained as follows:
Molecular mass of methane, (CH4) = (12.011 u) + 4 (1.008 u)
= 16.043 u
Similarly, molecular mass of water (H 2O) = 2 × atomic mass of hydrogen + 1 × atomic mass of
oxygen
= 2 (1.008 u) + 16.00 u
= 18.02 u
Formula Mass
Some substances, such as sodium chloride, do not contain discrete molecules as their constituent
units. In such compounds, positive (sodium ion) and negative (chloride ion) entities are arranged
in a three-dimensional
dimensional structure, as shown in Fig. 1.
Fig. 1 Packing of Na+ and Cl– ions in sodium chloride
,It may be noted that in sodium chloride, one Na+ ion is surrounded by six Cl– ion and vice-versa.
The formula, such as NaCl, is used to calculate the formula mass instead of molecular mass as in
the solid state sodium chloride does not exist as a single entity.
Thus, the formula mass of sodium chloride is atomic mass of sodium + atomic mass of chlorine
= 23.0 u + 35.5 u = 58.5 u
Stoichiometry and Stoichiometric Calculations
The word ‘stoichiometry’ is derived from two Greek words — stoicheion (meaning, element)
and metron (meaning, measure). Stoichiometry, thus, deals with the calculation of masses
(sometimes volumes also) of the reactants and the products involved in a chemical reaction.
Stoichiometry is an important concept in chemistry that helps us use balanced chemical
equations to calculate amounts of reactants and products. Before understanding how to calculate
the amounts of reactants required or the products produced in a chemical reaction, let us study
what information is available from the balanced chemical equation of a given reaction. Let us
consider the combustion of methane. A balanced equation for this reaction is as given below:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
Here, methane and dioxygen are called reactants and carbon dioxide and water are called
products. Note that all the reactants and the products are gases in the above reaction and this has
been indicated by letter (g) in the brackets next to its formula. Similarly, in case of solids and
liquids, (s) and (l) are written respectively.
The coefficients 2 for O2 and H2O are called, stoichiometric coefficients. Similarly the
coefficient for CH4 and CO2 is one in each case. They represent the number of molecules (and
moles as well) taking part in the reaction or formed in the reaction.
Limiting Reagent
Many a time, reactions are carried out with the amounts of reactants that are different than the
amounts as required by a balanced chemical reaction. In such situations, one reactant is in more
amount than the amount required by balanced chemical reaction. The reactant which is present in
the least amount gets consumed after sometime and after that further reaction does not take place
,whatever be the amount of the other reactant. Hence, the reactant, which gets consumed first,
limits the amount of product formed and is, therefore, called the limiting reagent.
Reactions in Solutions
A majority of reactions in the laboratories are carried out in solutions. Therefore, it is important
to understand as how the amount of substance is expressed when it is present in the solution. The
concentration of a solution or the amount of substance present in its given volume can be
expressed
ed in any of the following ways
ways:
1. Mass per cent or weight per cent (w/w %)
2. Mole fraction
3. Molarity
4. Molality
,
