Q:- What is empirical formula. Give examples.
Ans. The symbolic representation of a compound which tells us about simplest whole number ratio between atoms of elements in a compound. For example, the empirical formula of glucose is CH2O, sulphuric acid is H2SO4, sodium chloride is NaCl, water is H2O and benzene is CH.
Q:- Many compounds have same chemical formulas/molecular and empirical formulas?
Ans. There are some compounds which have same molecular and empirical formulas. For example, water (H2O), carbondioxide (CO2), ammonia (NH3), nitric acid (HNO3), sulphuric acid (H2SO4), sodium chloride (NaCl), calcium chloride (CaCl2) etc.
Note that for all ionic compounds, chemical formulas are same to their empirical formulas.
Q:- Define molecular mass. Give examples.
Ans. The sum of atomic masses of all atoms present in a molecule is called molecular mass. For example, the molecular mass of hydrogen (H2) is 2 amu (1amu + 1 amu = 2 amu), chlorine (Cl2) is 71 amu, (35.5 amu + 35.5 amu = 71 amu) water (H2O) is 18 amu (2 x 1 amu + 16 amu = 18 amu), and sulphuric acid (H2SO4) is 98 amu ( 2 x 1 amu + 32 amu + 4 x 16 amu = 98 amu).
The term molecular mass is only used for molecular elements and molecular compounds (covalent compounds) only.
Q:- Define formula mass. Give examples.
Ans. The sum of atomic masses of all atoms present in a formula unit of an ionic compound is called formula mass. For example, The formula mass of sodium chloride (NaCl) is 58.5 amu (23 amu + 35.5 amu = 58.5 amu) , calcium oxide (CaO) is 56 amu (40 amu + 16 amu = 56 amu) and calcium chloride (CaCl2) is 111 amu (40 amu + 2 x 35.5 amu = 111 amu).
The term formula mass is used for ionic compounds only.
Q:- Define formula unit. Give examples.
Ans. The arrays (arrangement) of oppositely charged ions in ionic compounds which show simplest ratio between cations and anions are called formula units. Ionic compounds contain formula units, not molecules or atoms.
For example, NaCl is the formula unit of sodium chloride which shows that it has one sodium ion (Na+) for one chloride ion (Cl–). Similarly, CaCl2 is the formula unit of calcium chloride which shows that it has one calcium ion (Ca2+) for two chloride ions (Cl–). Remember that onic compounds are shown by formula units.
Q:- Define molecule. Explain the types of molecules with examples.
Ans. The smallest particle of a molecular element or a molecular compound which can exist in free state (uncombined state) is called molecule. For example, Cl2 is the molecule of chlorine gas, H2O is the molecule of water. There are two types of molecules.
- Monoatomic molecules. These are molecules having one atom, e.g helium (He) and neon (Ne) etc. All noble gases contain monoatomic molecules.
- ii. Polyatomic molecules. These are molecules containing more than one atom, e.g water (H2O) and table sugar (C12H22O11) etc.
Q:- Define chemical species. Give examples.
Ans. An atom, molecule of ion which is under chemical process of measurement is called chemical species. For example, a sample of sodium element, sodium chloride or a piece of ice etc.
Q:- Define ion. Give examples.
Ans. A charged species which is formed by loss or gain of electron from an atom of chemically bonded group of atoms. For example, calcium ion (Ca2+) and oxide ion (O2-) etc. There are two types of ions, i.e cation and anion.
Q:- What is cation and anion? How are they formed? Give examples.
Ans. i. Cation.
It is positively charged ion. It is formed from metal atoms by losing one or more electrons. For example potassium ion (K+) which is formed by losing one electron and aluminium ion (Al3+) which is formed by losing three electrons.
ii. Anion.
It is negatively charged ion. It is formed from non metal atoms by gaining one or more than one electrons. For example, fluoride ion (F–) which is formed by gaining one electron and oxide ion (O2–) which is formed by gaining two electrons.
Q:- Why an ion acquires (adopts) a net positive and negative charge? Give examples.
Ans. A cation gets a positive charge because after losing electron/s because the number of electrons decreases than the number of protons, so an overall positive charge appears on it.
Sodium atom (2311Na) = 11 electrons (-11 charge) + 11 protons (+11)
= -11 + (+11) = 0
It means no charge appears on sodium atom.
If sodium loses one electron, it means the number of electrons decreases by one unit than the number of protons (proton number does not change), so a net positive charge appears on it as follows.
Sodium ion (2311Na1+) = 10 electron (-10 charge) + 11 protons (+11 charge)
= -10 + (+11) = +1
It means +1 charge appears on sodium ion.
On the other hand, an anion gets negative charge after gaining electron/s because the number of electrons increases than the number of protons, so an overall negative charge appears on it.
Chlorine atom (3517Cl) = 17 electrons (-17 charge) + 17 protons (+17)
= -17 + (+17) = 0
It means no charge appears on chlorine atom.
If chlorine atom gains one electron, it means the number of electrons increases by one unit than the number of protons (proton number does not change), so a net negative charge appears on it as follows.
Chloride ion (3517Cl1-) = 18 electron (-18 charge) + 17 protons (+17 charge)
= -18 + (+17) = -1