Unit 15: Stoichiometry — Long Questions

Key points Molar volume | Measurement of Volume at Room temperature and Pressure (RTP)

Molar volume
It has been found that one mole of all gases occupies a volume of 24 dm³ at room temperature and pressure (RTP). This is called molar volume.
Examples: 24dm³ of any gas at RTP = 1 mole
24dm³ of H₂ gas at RTP = 1 mole
24dm³ of CH₄ gas at RTP = 1 mole

Formula Number of moles of gas = Volume of the gas at RTP / Molar volume of the gas

Measurement of volume at room temperature and pressure (RTP)
The volume of a gas varies with changes in pressure and temperature. In order to compare the volumes occupied by different gases, we must adopt a set of standard conditions of temperature and pressure called room temperature and pressure (RTP). Accordingly, 25°C (298.15K) and one atmosphere (760 torr) are used as standard conditions of temperature and pressure for measurement of volumes of gases at room temperature.
Explanation: It is worth noting that one mole of each gas contains the same number of molecules, occupies the same volume and at the same time has different mass. In other words, the masses and the sizes of the molecules of gases do not change the volume occupied by them.
Importance of molar volume: The concept of molar volume is useful because it enables us to calculate the number of moles or masses of the gases if we know their volumes.

Key points Concentration of a solution | Calculation of concentration of solution

Concentration of a solution
Concentration of a solution is a measure of the amount of solute that has been dissolved in a given amount of solution or solvent.
Amount of solute: The amount of solute dissolved is expressed either in grams or in moles.
Quantity of solution: The quantity of solution is expressed in terms of its volume, dm³ or cm³
Unit: The unit used for the mass/volume concentration is then g/dm³ while for the molar concentration it is mol/dm³.

Calculation of concentration of solution
Concentration of solution is calculated by following formula:
Concentration of a solution = Mass of solute in gram / Volume of solution in dm³

Key point Calculation of the Concentration of a Solution in Titration Using Empirical Data

Calculation of the concentration of a solution in titration using empirical data
The concentration of a solution can be determined by titrating it with a solution of known concentration till the end point is reached. For this purpose, you need to know the balanced chemical equation for the reaction between the two solutions, the volumes of the two solutions used during the titration and the concentration of the other solution.

Key points Percentage Composition by Mass | Importance of Percentage Composition

Percentage composition by mass
Percentage composition of a compound refers to the percentage mass of each element present in the compound.
Percentage of an element: The percentage of an element in a compound is the number of grams of that element present in 100 grams of the compound.
Formula: The percentage of an element can be determined by applying the following formula.
Percentage of an element = Mass of the element in the compound / Formula mass of the compound ×100

Importance of percentage composition

• Percentage composition is a useful concept to determine the purity of a compound and its empirical and molecular formulae.

• Percentage composition of a compound can be determined both theoretically as well as by performing quantitative analysis.

Key points Empirical Formula | Empirical Formulae of Ionic Compounds | Steps to Calculate Empirical Formula of a Compound

Empirical formula
The formula which shows the simplest whole number ratio of atoms present in a compound is called empirical formula.

Examples Empirical formula of hydrogen peroxide (H₂O₂) is HO, that of water (H₂O) is H₂O and for benzene (C₆H₆) is CH.

Empirical formulae of ionic compounds
• All the ionic compounds are represented by their empirical formulae. These formulae show the simplest ratio present between their ions.
• The formula of sodium chloride(NaCl) represents the simplest ratio between sodium and chloride ions present in it.
• Similarly the formula of calcium chloride (CaCl₂) shows the ratio present between calcium and chloride ions.

Similar empirical formulae of different compounds: Since the empirical formula does not tells about the actual number of atoms present in that compound rather it represents simplest or minimum ratio between atoms. It is possible that some compounds may have same empirical formula.

Examples Both acetylene (C₂H₂) and benzene (C₆H₆) have same empirical formula CH. In the same way acetic acid (CH₃COOH) and glucose (C₆H₁₂O₆) also have the same empirical formula CH₂O.

Steps to calculate empirical formula of a compound
Empirical formula of a compound can be calculated following the steps shown below.
(i) Percentage composition: Determine the percentage composition of the compound.
(ii) Number of moles: Find out the number of moles of each element. This is done by dividing the mass of each element (percentage of element) by its atomic mass.
(iii) Atomic ratio: Determine the atomic ratio of each element in the compound. This ratio can be found by dividing the number of moles of each element by the smallest number of moles.
(iv) Whole number atomic ratio: If the atomic ratio obtained is simple whole number, it gives the empirical formula, otherwise multiply this ratio with suitable digit to get the whole number ratio.
(v) Subscripts for the elements: Use this whole number ratio as a subscript for the elements to get the empirical formula.

Key points Molecular Formula | Calculation of Molecular Formula from Empirical Formula

Molecular formula
Molecular formula of an element or a compound represents the actual number of atoms present in the molecule of these substances.

Examples Water, hydrogen peroxide, ethylene and sulphur have molecular formulae H₂O, H₂O₂, C₂H₄ and S₈ respectively.

Calculation of molecular formula from empirical formula
Molecular formula of a compound can be calculated by multiplying a whole number with the empirical formula of that compound as shown in the following equation.
Molecular formula = n (Empirical formula)
n = Molecular Mass / Empirical Formula Mass

Key points Chemical Equation | Complete and Balanced Chemical Equation

Chemical equation
Symbolic representation of a chemical reaction in terms of symbols of elements and formulae of compound is called chemical equation.

Complete and balanced chemical equation
A complete and balanced chemical equation tells us the mole ratio or mass ratio between the reactants and the products. With the help of this ratio we can find out the masses of the products from the given masses of the reactants or the masses of the reactants from the given masses of the products.

Example The following equation tells us that one mole (100 g) calcium carbonate reacts with two moles (73 g) of hydrochloric acid to produce one mole (111 g) of CaCl₂, one mole (18 g) of water and one mole (44 g) of carbon dioxide.

CaCO₃₍ₛ₎ + 2HCl₍ₐᵩ₎ ———> CaCl₂₍ₐᵩ₎ + H₂O₍ₗ₎ + CO₂₍ᵍ₎
1 mole 2 moles 1 mole 1 mole 1 mole
100 g 73 g 111 g 18 g 44 g

Key points Limiting Reactant | Identification of a Limiting Reactant

Limiting reactant
The reactant which is consumed earlier is called the limiting reactant. It can also be defined as the reactant that produces the lowest moles of products in a reaction. In fact, it is the limiting reactant which controls the amount of the product formed in a chemical reaction.

Non limiting reactant The reactant which is in excess and is left unreacted after the reaction is complete. is called non-limiting reactant or excess reactant.

Explanation
• In a chemical reaction, reactants are not always mixed according to stoichiometric ratio shown in the balanced chemical equation.
• Most often in experimental works, one or more reactants is /are used in large excess. This is done deliberately to ensure that the other expensive reactant is used completely.
• In this way the reactant which is used in excess is left behind at the end of the reaction and the other reactant is consumed completely.

Identification of a limiting reactant
A limiting reactant present in a chemical reaction can be identified by carrying out the following steps.
(i) Number of moles of reactant: Calculate the number of moles of the reactants from the amounts provided.
(ii) Number of moles of products: Calculate the number of moles of the product which will form with the given moles of reactants.
(iii) Least amount of product: Identify the reactant which gives the least amount of the product. This reactant will be the limiting reactant.

Key points Yield | Percentage Yield

Yield
There are two types of yield that is actual yield and theoretical yield.
(i) Actual yield: The quantity of the product obtained when chemical reaction is performed is called the actual yield. This yield is always less than the theoretical yield.
(ii) Theoretical yield: The theoretical yield is the amount of product which we calculate assuming that all the reactants react according to the balanced chemical equation.

Percentage yield / Efficiency of the reaction
A chemist is usually interested in the efficiency of the reaction which is commonly expressed by comparing the actual yield with the theoretical yield in the form of percentage yield. Percentage yield is calculated by applying the following formula.

Percentage yield = (Actual yield / Theoretical yield) × 100

Key points Percentage Purity | Calculation of Percentage Purity

Percentage purity
In chemistry, purity refers to the degree to which a substance is free from impurities. Percentage purity of compound indicates the amount of pure compound present in an impure sample.

Calculation of percentage purity
Percentage purity of a sample is calculated by the following formula.

Percentage purity = (Mass of the pure sample / Total mass of the impure sample) × 100