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Most asked questions -
1. Mole concept
2. Laws of Chemical combination
This chapters has following important topics -
In this unit, you'll study about -
• Characteristics of three states of matter
• Classification of different substances into elements, compounds and mixtures
• Use of scientific notations and determining significant figures
• Precision and accuracy;
• SI base units and conversion of physical quantities from one system of units to another
• Laws of chemical combination
• Significance of atomic mass, average atomic mass, molecular mass and formula mass
• Mole and Molar mass;
• Calculation of mass percent of component elements constituting a compound
• Empirical formula and molecular formula
• Stoichiometry
The study of chemistry is very important as its domain encompasses every sphere of life. Chemists study the properties and structure of substances and the chancres undergone by them.
All substances contain matter which can exist in three states - solid, liquid or gas. The constituent particles are held in different ways in these states of matter and they exhibit their characteristic properties.
Matter can exist as elements, compounds or mixtures. An element contains particles of only one type which may be atoms or molecules. The compounds are formed where atoms of two or more elements combine in a fixed ratio to each other. Mixtures occur widely and many of the substances present around us are mixtures.
Molecular mass is the sum of atomic masses of the element present in a molecule. For example in methane (CH4) the atomic mass of carbon is 12 and there is one carbon only, so 12×1 = 12. Similarly the atomic mass of hydrogen is 1 and since there are four hydrogen atoms, so 1×4 = 4. Show the molecular mass of methane will be 12 + 4 = 16.
The molecular formula in case of an ionic compound is also known as formula mass.
An empirical formula represent the simplest whole number ratio of various atoms present in a compound while the molecular formula will show the exact number of different type of atoms present in a molecule.
Stoichiometry deals with the calculation of volume, mole or mass of the product and the reactant involved in a chemical reaction.
When the properties of a substance are studied, measurement is an important part. The quantification of properties requires a system of measurement and units in which the quantities are to be expressed.
The scientific community has agreed to have a uniform and common system throughout the world which is abbreviated as SI units (International System of Units).
Since measurements involve recording of data which are always associated with a certain amount of uncertainty, the proper handling of data obtained by measuring the quantities is very important.
The uncertainty is taken care of by specifying the number of significant figures in which the observations are reported. The dimensional analysis helps to express the measured quantities in different systems of units. Hence, it is possible to interconvert the results from one system of units to another.
The combination of different atoms is Governed by basic laws of chemical combination - these being the Law of Conservation of Mass, Law of Definite Proportions, Law of Multiple Proportions, Gay Lussac's Law of Gaseous Volumes and Avogadro Law. All these laws led to the Dalton's atomic theory which states that atoms are building blocks of matter.
The atomic mass of an element is expressed relative to 12C isotope of carbon which has an exact value of 12u. Usually, the atomic mass used for an element is Average Atomic mass. It is average atomic mass obtained by taking into account the natural abundance of different isotopes of that element. The molecular mass of a molecule is obtained by taking sum of the atomic masses of different atoms present in a molecule.
The molecular formula can be calculated by determining the mass per cent of different elements present in a compound and its molecular mass. The number of atoms, molecules or any other particles present in a given system are expressed in the terms of Avogadro constant (6.022 x 10^23) . This is known as 1 mol of the respective particles or entities.
Chemical reactions represent the chemical changes undergone by different elements and compounds. A balanced chemical equation provides a lot of information. The coefficients indicate the molar ratios and the respective number of particles taking part in a particular reaction.
The quantitative study of the reactants required or the products formed is called stoichiometry. Using stoichiometric calculations, the amounts of one or more reactant(s) required to produce a particular amount of product can be determined and vice-versa.
In a reaction, the reactant which gets consumed and hence limits the product formed and their four is known as limiting reagent.
The amount of substance present in a given volume of a solution is expressed in number of ways, e.g., mass per cent, mole fraction, molarity and molality.
Some Previous year questions (PYQs) of Chapter 1 Some basic Concepts of Chemistry from NEET and AIIMS and AIPMT are -
1. Equal masses of H2, O2 and methane have been taken in a container of volume V at temperature 27 °C in identical conditions. The ratio of the volumes of gases H2 : O2 : methane would be
(a) 8 : 16 : 1 (b) 16 : 8 : 1
(c) 16 : 1 : 2 (d) 8 : 1 : 2
2. What volume of oxygen gas (O2) measured at 0°C and 1 atm, is needed to burn completely 1 L of propane gas (C3H8) measured under the same conditions?
(a) 5 L (b) 10 L
(c) 7 L (d) 6 L
3. A 0.24 g of a volatile gas, upon vaporisation, gives 45 mL vapour at NTP. What will be the vapour density of the substance? (Density of H2 = 0.089 g/L)
(a) 95.93 (b) 59.93
(c) 95.39 (d) 5.993
4. The molecular weight of haemoglobin is approximately 67200. The number of iron atoms (Atomic weight of Fe is 56) present in one molecule of haemoglobin is
(a) 4 (b) 6
(c) 3 (d) 2
5. The number of moles of oxygen in one litre of air containing 21% oxygen by volume, under standard conditions, is
(a) 0.0093 mol (b) 2.10 mol
(c) 0.186 mol (d) 0.21 mol
6. Which has the maximum number of molecules among the following?
(a) 44g CO2. (b) 48g O3
(c) 8g H2. (d) 64g SO2
7. The number of atoms in 0.1 mol of a triatomic gas is
(a) 6.026 × 10^22 (b) 1.806 × 10^23
(c) 3.600 × 1023 (d) 1.800 × 1022
8. The number of gram molecules of oxygen in 6.02 × 10^24 CO molecules is
(a) 10 g molecules (b) 5 g molecules
(c) 1 g molecule (d) 0.5 g molecules
9. Ratio of Cp and Cv of a gas ‘X’ is 1.4. The number of atoms of the gas ‘X’ present in 11.2 litres of it at NTP will be
(a) 6.02 × 10²³ (b) 1.2 × 10²³
(c) 3.01 × 10²³ (d) 2.01 × 10²³
10. The number of oxygen atoms in 4.4 g of CO2 is
(a) 1.2 × 10²³ (b) 6 × 10²³
(c) 6 × 10²⁴ (d) 12 × 10²³
11. The maximum number of molecules is present in
(a) 15 L of H2 gas at STP
(b) 5 L of N2 gas at STP
(c) 0.5 g of H2 gas
(d) 10 g of O2 gas
12. Which has maximum molecules?
(a) 7g N2 (b) 2g H2
(c) 16g NO2 (d) 16g
21. The number of atoms in 4.25 g of NH3 is
approximately
(a) 4 × 1023 (b) 2 × 1023
(c) 1 × 1023 (d) 6 × 1023
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