# 18 Important Chemistry Questions and Answers

If you are looking for some important questions of Chemistry then you are at the right place.

Here are some important questions of Chemistry for you.

## Q.1- What is the SI unit of mass? How is it defined?

Solution: The SI unit of mass is kilogram (kg). Kilogram is defined as the mass of platinum-iridium (Pt-Ir) block, stored at the International Bureau of Weights and Measures in France. Thus it is the mass of the international prototype of the kilogram.

## Q.2- What do you mean by significant figures?

Solution:  The significant figures in a number are all the certain digits plus on doubtful digits. It depends upon the precision of the scale or instrument used for the measurement. For example, if volume of a liquid is reported to the 18.25 mL, the digits 1, 8 and 2 are certain while 5 is doubtful. So, it has four significant figures ( three certain plus one doubtful).

## Q.3- What is the symbol for SI unit of mole? How is the mole defined ?

Solution: Symbol for SI unit of mole is mol. It is defined as the amount of a substance that contains as many particles or entities as there are atoms in 12 g of 12C isotope.

## Q.4- What is the difference between molality and molarity ?

Solution: Molality is the number of moles of solute present in one kilogram of solvent. Whereas molarity is the number of moles solute dissolved liter of solution.

Molality is independent of temperature whereas molality depends on temperature.

## Q.5- In the combustion of methane in air, what is the limiting reactant and why?

Solution: Methane is the limiting reactant because the other reactant is oxygen of the air which is always present in excess. Thus, the amount of carbon dioxide and water formed will depend upon the amount of CH2 burnt

Since one molecule of methane contains 10 electrons and therefore, 1 kg mole of CH4 contains 10 * 6.022 * 1036 = 6.022 * 1027 electrons.

## Q.6- Which aqueous solution has higher concentration: 1 molar solution of the same solute. Give reason.

Solution:  1 molar aqueous solution has higher concentration than 1 mole solution.

A mole solution contains one mole of solute in one litre of solution while a one molal solution contains one mole of solute in 1000 g of solvent.

If density of water is 1, then on mole of  solute of present in 1000 mL of water in 1 mole solution while one mole of solute is present in less than 1000 mL of water in 1 mole solution ( 1000 mL solution = amount of solute + amount of solvent). Thus, 1 molar  solution is more concentrated.

## Q.7- Will the molarity of a solution at 500C be same, less or more than molarity at 250C?

Answer: Molarity at 500C of a solution will be less than that at 250C because molarity decreases with increase in temperature. This is because volume of the solution increases with increase in temperature. But number of number of moles of solution remain the same.

## Q.8- Is the law of constant composition true for all types of compounds? Explain why or why not.

Answer: No, law of constant composition is not true for all types of compounds. It is true for only those compounds which are obtained from one isotope. For Example, carbon exists in two common isotopes; 12C and 14C. When it forms 12CO2, the ratio of masses is 12:32 or 3:8. However, when it is formed from 14C i.e., 14CO2, the ratio will be 14:32 i.e., 7:16, which is not same as in the first case.

## Q.9- Why is molality preferred over molarity in expressing the concentration of a solution?

Answer: Molality is the number of moles of a solute present in 1000g of the solvent. While molarity is the number of moles of solute present in 1000mL of the solution. Thus, molality involves only masses which do not change with temperature whereas molarity involves volume which changes with temperature. Hence molality is preferred over molarity.

## Q.10- What is the difference in expressing a weight of a solid as 36.5*103g and 36.50*103  g.

Answer: 36.5*103g has three significant figures while 36.50*103 has four significant figures. Hence 36.50 represents greater accuracy than 36.5.

Answer: Avogadro’s law stated that under similar conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules.

## Q.12- What do you mean by significant figures?

Answer: In a given number, all the certain digits plus one doubtful digit correspond to significant figures. They depend on the precision of the scale or the instrument used for the measurement. For example, the number 1.587 represents four significant figures (three are certain and one is doubtful).

## Q.13- In Rutherford’s experiment, generally the thin foil of heavy atoms, like gold, platinum, etc. have been used to be bombarded by the alpha-particles. If the thin foil of light atoms like aluminium, etc. is used, what differences would be observed from the above result?

Answer: In Rutherford’s experiment heavy metals have heavy nucleus carrying a large amount of positive charge. Therefore, some alpha-particles even got deflected back after hitting the nucleus. Because of large positive charge on the heavy nucleus, some alpha particles are deflected through small angles which passed closer to the nucleus because of repulsion. However, if lighter atoms are used, their nuclei will be light and they will have small positive charge on the nucleus. As a result, the number of particles deflected back and those deflected through small angles will be very very small, almost negligible.

## Q.14- Symbols 79Br35 and 79Br can be written, whereas symbols 35Br79 and 35Br are not acceptable. Answer Briefly.

Answer: Atomic number of an element (say Br) is fixed. But its mass number is not fixed depending upon the isotope. Hence, it is necessary to indicate mass number.

## Q.15- An ion with mass number 37 possesses one unit of negative charge. If the ion contains 11.1% more neutrons than the electrons, find the symbol of the ion.

Answer: Since it carries -1 charge, it will have one electron more than the number of protons.

## Q.16- The Bromine atom possesses 35 electrons. It contains 6 electrons in 2p orbital, 6 electrons in 3p orbitals and 5 electrons in 4p orbitals. Which of these electrons experiences the lowest effective nuclear charge?

Answer: 4p electrons are farthest from the nucleus and therefore, these electrons will experience the lowest effective nuclear charge.

## Q.17- Arrange s, p and d subshells of a shell in the increasing order of effective nuclear charge (Zeff) experienced by the electron present in them.

Answer: s- orbitals shield the electrons from the nucleus more effectively than p-orbitals which is turn shield more effectively than d-orbitals. Hence, the subshells in the increasing order of effective nuclear charge is:

d < p < s

## Q.18- What is the experimental evidence in support of idea that electronic energies in an atom are quantized?

Answer: In the line spectrum of any element, lines of definite wavelength are obtained. These lines correspond to electronic transitions between fixed energy levels. Thus, the electros in these energy levels have definite energy i.e., quantized values.

So, these are the some important questions of chemistry. We hope that you like the article.