Atoms, Molecules and Chemical Arithmatics NIOS Intext Questions With Answer
National Institute of Open Schooling (NIOS)
Sr. Secondary, Module-1
Lession-1: Atoms, Molecules and Chemical Arithmatics Intext Questions With Answer
INTEXT QUESTIONS 1.1
1. Chemistry plays a vital role in many areas of science and technology. What are those areas?
Answer: Chemistry plays a pivotal role in many areas of science and technology e.g. in health, medicine, energy and environment, food, agriculture and new materials.2. Who proposed the particulate nature of matter?
Answer: Thus, particulate nature (i.e. state of matter with two forms of matter combined which is a solid and a liquid) of matter was proposed by the English scientist John Dalton.3. What is law of conservation of mass?
Answer: The law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants.4. What is an atom?
Answer: Atoms are defined as the basic building blocks of matter. It is the smallest constituent unit of matter that possess the properties of the chemical element. Atoms don't exist independently, instead, they form ions and molecules which further combine in large numbers to form matter that we see, feel and touch.5. What is a molecule?
Answer: A group of two or more than two atoms of the same or different elements that are chemically bonded together is called a molecule.For example, two atoms of hydrogen and one atom of oxygen react with each other and form one molecule of water.
6. Why is the symbol of sodium Na?
Answer: The symbol Na derives from the Latin word 'natrium' for "natron" (soda in English).7. How is an element different from a compound?
Answer: An element is made of only one kind of atom and a compound is made of two or more kinds of atoms.Example: H2 and N2 molecules are an element as it is formed from only one type of atom. H2O is a compound as it is formed from two different types of atoms H and O.
INTEXT QUESTIONS 1.2
1. Name the SI Unit of mass
Answer: SI Unit of mass is Kilogram2. What symbol will represent 1.0 × 10–6 g ?
Answer: The symbol representing the quantity 1.0 × 10–6 g will be μ and the value will be 1.0 μg.3. Name the prefixes used for (i) 102 and (ii) 10–9
Answer: (i) The prefix used for the representation of quantity 102 is hecta.(ii) The prefix used for the representation of quantity 10–9 is nano.
4. What do the following symbols represent?
(i) Ms (ii) ms
Answer: (i) The Ms symbol represent Mega second 106 s.(ii) The ms symbol represent millisecond 10–9 s.
INTEXT QUESTIONS 1.3
1. A sample of nitrogen gas consists of 4.22 × 1023 molecules of nitrogen. How many moles of nitrogen gas are there?
Answer: (4.22 × 1023 moles of N2 X 1 mole of N2 ) / 6.022 × 1023 moles of N2 = 0.71 moles of N22. In a metallic piece of magnesium, 8.46 × 1024 atoms are present. Calculate the amount of magnesium in moles.
Answer: Amount of magnesium (moles) = 8.46 × 1024 atoms / 6.022 × 1023 atoms per mole = 14.05 mol.3. Calculate the number of Cl2 molecules and Cl atoms in 0.25 mol of Cl2 gas.
Answer: No. of Cl2 molecules in 0.25 mol of Cl2= 0.25 × 6.022 × 1023 molecules
= 1.5055 × 1023 molecules
Since each Cl2 molecule has two Cl atoms
so, the number of Cl atoms are
= 2 × 1.5055 × 1023
= 3.011 × 1023 atoms
INTEXT QUESTIONS 1.4
1. Calculate the molar mass of hydrogen chloride, HCl.
Answer: Molar mass of hydrogen chloride= molar mass of HCl
= 1 mole of H + 1 mol of Cl
= 1.0 g mol–1 + 35.5 g mol–1
= 36.5 g mol–1
2. Calculate the molar mass of argon atoms, given that the mass of single atom is 6.634 × 10–26 kg.
Answer: Molar mass of argon atom= mass of 1 mole of argon
= mass of 6.022 × 1023 atoms of argon
= 6.634 × 10–26 kg × 6.022 × 1023 mol–1
= 39.95 × 10–3 kg mol–1
= 39.95 g mol–1
3. Calculate the mass of 1.0 mol of potassium nitrate, KNO3 (atomic masses: K = 39 amu; N = 14 amu, O = 16 amu).
Answer: Molar mass of KNO3= mass of 1 mol of K + mass of 1 mol of N + mass of 3 mol of O
Since molar mass of an element is numerically equal to its atomic mass, but has the unit of g mol–1 in place of amu
= 39.1 g + 14.0 g + 3 × 16.0 g so, the Molar mass of KNO3
= 39.1 g + 14.0 g + 48.0 g
= 101.1 g mol–1
4. The formula of sodium phosphate is Na3PO4. What is the mass of 0.146 mol of Na3PO4? (atomic masses : Na = 23.0 amu, P = 31.0 amu; O = 16.0 amu).
Answer: Mass of 1 mole of Na3PO4= 3 × (mass of 1 mol of Na) + mass of 1 mole of P + 4 × (mass of 1 mol of oxygen)
= 3 (23.0 g) + 31.0 g + 4 (16.0 g)
= 69.0 g + 31.0 g + 64.0 g
= 164.0 g
so, the Mass of 0.146 mole of Na3PO4
= 0.146 × 164.0 g
= 23.94 g
INTEXT QUESTIONS 1.5
1. How many moles of Cu atoms are present in 3.05 g of copper (Relative atomic mass of Cu = 63.5).
Answer: Moles of Cu atoms in 3.05 g copper = 3.05 g / 63.5 g mol–1= 0.048 mol
2. A piece of gold has a mass of 12.6 g. How many moles of gold are present in it? (Relative atomic mass of Au = 197)
Answer: Moles of Au = 12.6 g / 197 g mol–1= 0.064 mol
3. In a combustion reaction of an organic compound, 2.5 mol of CO2 were produced. What volume would it occupy at STP (273K, 1bar) ?
Answer: Molar volume of any gas at STP (273 K, 1 bar) = 22.7 Lso, the volume occupied by 2.5 mol CO2 at STP = 2.5 × 22.7 L
= 56.75 L
INTEXT QUESTIONS 1.6
1. For the compound Fe3O4, calculate percentage of Fe and O.
Answer: Molar mass of Fe3O4= 3 × 56 + 4 × 16
= (168 + 64)
= 232 g mol–1
Percentage of Fe = (168 × 100) / 232 = 72.41%
Percentage of O = (64 × 100) /232 = 27.59%
2. State percent composition for each of the following:
(a) C in SrCO3 (b) SO3 in H2SO4
Answer: (a) Molar mass of SrCO3 = 87.6 + 12.0 + 48.0= 147.6 g mol–1
Percentage of carbon C in SrCO3 = (12 × 100) / 147.6 = 8.13%
(b) Molar mass of H2SO4 = 2.0 + 32.1 + 64.0
= 98.1 g mol–1
Molar mass of SO3 = 32.1 + 48.0
= 80.1 g mol–1
Percentage of SO3 in H2SO4 = 80.1 × 100) / 98.1
= 81.65%
3. What are the empirical formulae of substances having the following molecular formulae?
H2O2, C6H12, Li2CO3, C2H4O2, S8, H2O, B2H6, O3, S3O9, N2O3
Answer:
| Substance | Emperical Formula |
|---|---|
| H2O2 | HO |
| C6H12 | CH2 |
| Li2CO3 | Li2CO3 |
| C2H4O2 | CH2O |
| S8 | S |
| H2O | H2O |
| B2H6 | BH3 |
| O3 | O3 |
| S3O9 | SO3 |
| N2O3 | N2O3 |
4. A compound is composed of atoms of only two elements, carbon and oxygen. If the compound contain 53.1% carbon, what is its empirical formula.
Answer: Percentage of carbon = 53.1%
Percentage of Oxygen = 46.9%
Suppose we take 100 g of the substance then moles of carbon = 53.1 / 12.0 g = 4.43 mol
mole of oxygen = 46.0 / 16.0 = 2.93 mol
molar ratio of C and O = (4.43/2.93) : (2.93/2.93)
= 1.50 : 1 or 3 : 2
Empirical formula of the compound is C3O2
