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Q1. Classify the following substances as elements, compounds and mixtures. In case of mixtures clearly indicate whether the mixture is homogenous or heterogeneous and also explain how would you separate them? a) Iron b) Salt and water c) Glass powder, iron filings and sugar d) Distilled water

Solution

a) Iron is an element b) Salt and water are a homogenous mixture which can be separated by distillation, in this process salt remains as residue. c) Glass powder, iron filing and sugar are a heterogeneous mixture. To separate the constituents dissolve the mixture in water, sugar dissolves. Now filter to separate undissolved glass powder and iron filings. Glass powder and iron filings can be separated with the help of a magnet. Sugar can be recovered from solution by evaporation. d) Distilled water is a compound.

Q2. Explain Law of Combining Volumes in detail.

Solution

Gay-Lussac’s Law of Gaseous Volumes (Law of Combining Volumes)

• This law was given by Gay-Lussac in 1808. He investigated a large number of chemical reactions occurring in gases.

• As a result of his experiments, Gay-Lussac found that there exists a definite relationship among the volumes of gaseous reactants and products.

Consider the following illustrations.

• When gases react together to produce gaseous products, the volumes of reactants and products bear a simple whole number ratio with each other, provided volumes are measured at same temperature and pressure.

Example 1: Under identical conditions of temperature and pressure equal volumes, say 1 litre i.e., 1L of each hydrogen and chlorine gases react together to produce double the volume, 2L of hydrogen chloride gas.   Thus, the ratio of volumes is 1 : 1 : 2. Example 2: Under identical conditions of temperature and pressure 2l of hydrogen gas reacts with 1L of oxygen gas to produce 2L of steam (water vapour).   Thus, the ratio of volumes is 2 : 1 : 2. Example 3: Under identical conditions of temperature and pressure 1L of nitrogen gas reacts with 3L of hydrogen to produce 2L of ammonia gas.   Here the ratio of volumes is 1 : 3 : 2. Conclusion: All examples cited above indicate that during gaseous reaction, gaseous reactants and products bear a simple ratio of whole number of volumes with each other.

Q3. Carbon forms two gaseous oxides. One of these oxides contains 42.8% carbon while the other contains 27.27% carbon. Explain which law of chemical combination this data illustrates. State this law.

Solution

The ratio of oxygen in the two compounds is 1:2 so this compound follows law of multiple proportions. According to this law when two elements combine with each other to form two or more than two compounds, the masses of one of the elements which combine with fixed mass of the other, bear a simple whole number ratio to the other.

Q4. Write a short note on formula unit mass.

Solution

Formula Mass

• Mass of a molecule of an ionic compound.
• It is  the sum of atomic masses of all the elements present in one formula unit of a compound.
• It is used for measuring the molecular mass of entities which do not exist in the solid form.

• Substances such as sodium chloride do not contain discrete molecules as their constituent units. In such compounds, positive (sodium) and negative (chloride) entities are arranged in a three-dimensional structure.

• In sodium chloride, one Na⁺ is surrounded by six Cl⁻ and vice versa.

Hence, in such cases, the formula mass is used to calculate the mass instead of the molecular mass. Just like the molecular mass, it is calculated by adding the atomic masses of the atoms present in one formula unit.

• Thus, formula mass of sodium chloride = atomic mass of sodium + atomic mass of chlorine

                                                                     = 23.0 u + 35.5 u                                                                      = 58.5 u

Q5. What were the limitations of Dalton's atomic theory? Explain in detail.

Solution

The main failures of Dalton atomic theory are:(1) It failed to explain how atoms of different elements differ from each other i.e. It did not tell anything about structure of the atom.(2) It could not explain how and why atoms different elements combine with each other to form compound atoms or molecules.(3) It failed to explain the nature of forces that bind together different atoms in a molecule.(4) It failed to explain Gay Lussac's law of combining volumes.(5) It did not make any distinction between ultimate particle of an element that takes part in reactions (atoms) and ultimate particle that has independent existence (molecule).

Q6. Why do atomic masses of most of the elements in atomic mass units involve fractions?

Solution

Atomic masses of most of the elements in atomic mass units involve fractions because atomic mass of an element is the average of relative masses of its various isotopes. While taking average, the result appears as a fraction.

Q7. How is empirical formula of a compound related to its molecular?

Solution

The empirical formula of a compound gives the simplest whole number ratio of atoms of various elements present in the molecule of a compound. Molecular formula = Empirical formula x n.

Q8. Which law of chemical combinations is applied in balancing a chemical equation?

Solution

In balancing a chemical equation, law of conservation of mass is applied.

Q9. The molecular mass of methane and oxygen are 16 and 32 respectively. If one litre of methane at S.T.P contains N molecules, what will be the number of molecules in 5 L of oxygen at S.T.P?

Solution

According to Avogadro’s law equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules. So 5 L of oxygen at S.T.P contains 5N molecules. 

Q10. Name the two chemical substances used in air conditioning.

Solution

Liquid ammonia or liquid sulphur dioxide are used in air conditioning.