Fundamentals Of Chemistry Exercise Short Questions
Explore essential short questions designed to reinforce the fundamentals of chemistry. Perfect for practice and revision, these exercises cover key topics in chemistry, helping you grasp core principles with ease.
Q NO 1. How many elements are present in each of the following ?
a) HF and Hf
b) Co and CO
c) Si and SiO2
d) PoCl2 and POCl3
Answer:
a) HF stands for hydrofluoric acid. It is a chemical compound formed by the chemical combination of two elements, one is hydrogen and second is fluorine.
So, HF contains two elements.
While Hf is a symbol used for the specific element, name Hafnium.
So, Hf is the only one element.
b) Co is a symbol used for the representation of an element, name cobalt.
So, Co is the only one element.
While CO stands for Carbon monoxide and it is a toxic gas formed by the chemical combination of two elements, Carbon and oxygen.
So, CO contains two elements.
c) Si is a symbol used for the representation of element, name known as silicon.
So, Si is the only one element.
While SiO2 is a solid chemical compound formed by the chemical combination of two elements, Silicon and Oxygen.
Hence SiO2 contains two elements.
d) PoCl2 stands for polonium dichloride. It is an ionic compound (salt) formed by the chemical combination of two elements, Polonium and Chlorine.
So, PoCl2 contains two elements.
While POCl3 stands for Phosphoryl chloride. It is a colourless liquid which is formed by the chemical combination of three elements, Phosphorus, Oxygen and Chlorine.
Hence, POCl3 contains three elements.
Q NO 2. Cm is the chemical symbol for curium, named after the famous scientist Madam Curie. Why was not the symbol C, Cu or Cr used instead ?
Answer;
Cm is the chemical symbol for curium. Curium was discovered in 1944.
C is the chemical symbol used for carbon. Carbon was discovered in 1772.
Cu is the chemical symbol used for copper. Copper was discovered in 9000 BC.
She or is the chemical symbol used for the chromium chromium was discovered in 1797.
As carbon, copper and chromium were discovered before the curium, so these elements were named earlier and the symbols were used for carbon, copper and chromium. Due to late discovery of curium, it was assigned the symbol Cm. That’s why, the symbols C, Cu and Cr cannot be used for the representation of curium element.
Q NO 3. What is the atomic number of an element? How does it differ from the mass number ?
Answer;
Atomic number of an element is defined as “the number of protons present in the nucleus of an atom is called atomic number”.
An atom as a whole is neutral. It contains the same number of protons and electrons. Thus, atomic number can also be defined as “the number of electrons revolving around the nucleus in orbits is called atomic number”.
While mass number is defined as “Sum of the number of protons and the number of neutrons present inside the nucleus of an atom is called mass number”.
Atomic number of an atom is represented by Z while mass number of an atom is represented by A.
If the atomic number of an atom is given, the number of neutrons present in the nucleus cannot be calculated but if the mass number of an atom is given, the number of neutrons present in the nucleus of that atom can be easily calculated by the following relationship,
No. of neutrons = Mass number – No. of protons
Q NO 4. Students often mix up the following elements. Give the name for each element.
-
- Mg and Mn
- K and P
- Na and S
- Cu and Co
Answer;
- Mg is the chemical symbol used for the representation of magnesium. While Mn is the chemical symbol used for the representation of manganese element.
- K is the chemical symbol of potassium element while P is the chemical symbol of phosphorus element.
- Na is the chemical symbol used for sodium element while S is the chemical symbol used for sulphur element.
- Cu is the chemical symbol used for the representation of copper element while Co is the chemical symbol used for the representation of cobalt element.
Q NO 5. a) Classify the following molecules as monoatomic, diatomic, triatomic and polyatomic molecules.
H2O, N2, S8, He, HCl, CO2, Ar, H2SO4, C6H12O6
Answer;
H20 (Water) is a triatomic molecule because it is composed of two hydrogen atoms and one oxygen atom.
N2 (Nitrogen gas) is a diatomic molecule because it is composed of two nitrogen atoms.
S8 (sulphur) is a poly-atomic molecule because it is composed of eight sulphur atoms.
He (Helium) is a monoatomic molecule because it is composed of only one atom of helium.
HCl (Hydrochloric acid) is a diatomic molecule because it is composed of one hydrogen atom and one chlorine atom.
CO2 (Carbon dioxide) is a tri-atomic molecule because it is composed of one carbon atom and two oxygen atoms.
Ar (Argon) is a monoatomic molecule because it is composed of only one atom of argon.
H2SO4 (Sulfuric acid) is a poly-atomic molecule because it is composed of two hydrogen atoms, one sulphur atom and four oxygen atoms.
C6H12O6 (Glucose ) is a poly-atomic molecule because it is composed of six hydrogen atoms, twelve hydrogen atoms and six oxygen atoms.
b) Classify the following as cation, anion, molecular ion, free radical and molecule.
CH3+, O-², CH3•, CO+, CO2, Cl-, Mg+², CO3-², O2, Na+, C2H5O-¹, H2O, Cl2
Answer;
CH3+ is a molecular ion. As it contains positive charge, it is a cationic molecular ion.
O-² is a molecular ion. As it contains negative charge, it is anionic molecular ion.
CH3• is a free radical as it contains a single unpaired electron.
CO+ is a molecular ion. As it contains positive charge, it is a cationic molecular ion.
CO2 is a molecule.
Cl- is an anion formed by the gain of single electron.
Mg+² is a cation formed by the loss of two electrons.
CO3-² is a molecular ion. As it contains negative charge, it is an anionic molecular ion.
O2 is a molecule.
Na+ is a cation which is formed by the loss of one electron.
C2H5O-¹ is a molecular ion. As it contains negative charge, it is an anionic molecular ion.
H2O is a molecule.
Cl2 is a molecule.
Q No. 6. Calculate the number of moles of butane (C4H10) in 151 grams of butane.
( At. Masses C= 12 amu and H= 1 amu).
Solution;
Mass ( in grams) of C4H10 = 151 g
Molar mass of C4H10 = 4(12) + 10(1) = 48 + 10 = 58 g/mol
No. Of moles of C4H10 = Mass (grams) of C4H10 / Molar mass of C4H10
By putting values in the above formula,
No. Of moles = 151 g/ 58g/mol = 2.603 moles
So, 151 grams of C4H10 contains 2.603 moles.
Q NO 7. What is the mass of 5 moles of ice? (Atomic masses: H= 1 amu , O = 16 amu)
Solution;
Number of moles of ice (H2O) = 5 moles
Molar mass of ice (H2O) = 2(1) + 1(16) = 18 g/mol
Number of moles = Mass (in grams) of ice (H2O) / molar mass of ice (H2O)
By cross multiplication, we get,
Mass (grams) of ice (H2O) = number of moles of ice × molar mass of ice
By putting values in the above formula, we get,
Mass (grams) of ice= 5 × 18 = 90 g
So 5 moles of ice (H2O) is equal to 90 grams by mass.
Q NO 8. Calculate the number of molecules in 6.50 moles of CH4.
Solution;
Number of moles of CH4 = 6.50 moles
Avogadro’s number = 6.023 × 10²³
Number of moles = number of molecules of CH4 / Avogadro’s number
By cross multiplication, we get,
Number of molecules of CH4 = number of moles of CH4 × Avogadro’s number
By putting the values in the above formula, we get,
No. Of molecules of CH4 = 6.50 × 6.023 × 10²³ = 39.14 × 10²³ = 3.914 × 10²⁴ molecules
So, the number of molecules in 6.50 moles of CH4 are 3.914 × 10²⁴.
Q NO 9. Calculate the average atomic mass of lithium from the following data;
Isotopes = ⁶Li and ⁷Li
Natural abundance = ⁶Li (7.5 %) and ⁷Li (92.5 %)
Relative atomic mass = ⁶Li (6.0151 amu) and ⁷Li (7.0160 amu)
Solution;
Atomic mass of 1st isotope = 6.0151 amu
% abundance of 1st isotope = 7.5 %
Atomic mass of 2nd isotope = 7.0160 amu
% abundance of 2nd isotope = 92.5 %
From Formula,
Average atomic mass = atomic mass of 1st isotope × it’s % abundance/100 + atomic mass of 2nd isotope × it’s % abundance/100
By putting the values given in the above table into formula, we get,
Average atomic mass = 6.0151 × 7.5/100 + 7.0160 × 92.5/100
Average atomic mass = 45.11/100 + 648.98/100 = 0.4511 + 6.4898 = 6.940
So, the average atomic mass of lithium is 6.940 .
Q NO 10. Calculate the mass of 6.68 × 10²³ molecules of PCl3.
Solution;
Number of moles of PCl3 = 6.68 × 10²³
Avogadro’s number = 6.023 × 10²³
Molar mass of PCl3 = 1(31) + 3(35.5) = 31 + 106.5 = 137.5 g/mol
From Formula,
Number of moles = number of molecules of PCl3 /Avogadro’s number
By putting values in the above formula, we get,
No. Of moles = 6.68 × 10²³/6.023 × 10²³ = 1.1090 moles
To calculate the mass of PCl3, we have the formula,
No. Of moles of PCl3 = Mass (grams) of PCl3/ Molar mass of PCl3
From cross multiplication, we get,
Mass (grams) of PCl3 = No. Of moles of PCl3 × Molar mass of PCl3
By putting values in the above formula, we get,
Mass (grams) of PCl3 = 1.1090 mol × 137.5 g/mol = 152.5 g
So, the mass of 6.68 × 10²³ molecules of PCl3 is equal to 152.5 grams.
