Atoms and Molecules NCERT Solutions

 Question 1: A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

Answer: Percentage of boron in given compound $=\left(\frac{0.096}{0.24}\right)\times 100=40\%$
Percentage of oxygen in given compound $=\left(\frac{0.144}{0.24}\right)\times 100=60\%$

Question 2: When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

Answer: We know that atoms of different elements combine in definite proportions to make a particular compound. Hence, 3 g of carbon will always give 11 g of carbon dioxide.

Question 3: What are polyatomic ions? Give examples.

Answer: An ion with more than one atom is called a polyatomic ion. Examples: Sulphate (SO42-), Nitrate (NO32-)

Question 4: Write the chemical formulae of the following.

(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.

Answer: (a) MgCl2, (b) CaO, (c) CuNO3, (d) AlCl3, (e) CaCO3

Question 5: Give the names of the elements present in the following compounds.

(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.

Answer: (a) Calcium, oxygen, (b) Hydrogen, bromine, (c) sodium, hydrogen, carbon, oxygen, (d) Potassium, sulphur, oxygen

Question 6: Calculate the molar mass of the following substances.

(a) Ethyne, C2H2

Answer: $2\times 12+2\times 1=26$ u

(b) Sulphur molecule, S8

Answer: $8\times 32=256$ u

(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)

Answer: $4\times 31=124$

(d) Hydrochloric acid, HCl

Answer: $1\times 1+1\times 35.5=36.5$ u

(e) Nitric acid, HNO3

Answer: $1\times 1+1\times 14+3\times 16=63$ u

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Question 7: What is the mass of:

(a) 1 mole of nitrogen atoms?

Answer: Atomic mass of nitrogen = 14 u
Hence, mass of 1 mole of nitrogen atoms = 14 g

(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?

Answer: Atomic mass of aluminium = 27 u
Hence, mass of 4 moles of aluminium atoms $=4\times 27=108$

(c) 10 moles of sodium sulphite (Na2SO3)?

Answer: Molecular mass of sodium sulphite
$=2\times 23+1\times 32+3\times 16=46+32+48=126u$
Hence, mass of 10 mole of sodium sulphite $=10\times 126=1260g$

Question 8: Convert into mole.

(a) 12 g of oxygen gas

Answer: Mass of 1 mole of oxygen = 32 g
Hence, number of mole in 12 g oxygen $=\frac{12}{32}=0.375M$

(b) 20 g of water

Answer: Molecular mass of water $=2\times 1+1\times 16=18u$
So, mass of 1 mole of water = 18 g
Hence, number of mole in 20 g of water $=\frac{20}{18}=1.11M$

(c) 22 g of carbon dioxide.

Answer: Molecular mass of carbon dioxide $=1\times 12+2\times 16=44u$
So, mass of 1 mole of carbon dioxide = 44 g
Hence, number of mole in 22 g of carbon dioxide $=\frac{22}{44}=0.5M$

Question 9: What is the mass of

(a) 0.2 mole of oxygen atoms?

Answer: Mass of 1 mole oxygen atoms = 16 g
Hence, mass of 0.2 mole of oxygen atoms $=0.2\times 16=3.2g$

(b) 0.5 mole of water molecules?

Answer: Mass of 1 mole of water = 18 g
Hence, mass of 0.5 mole of water $=0.5\times 18=9g$

Quesiton 10: Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.

Answer: Mass of 1 mole of sulphur $=8\times 32=256g$
Hence, number of mole in 16 g of sulphur $=\frac{16}{256}=0.625M$

Question 11: Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)

Answer: Formula of aluminium oxide = Al2O3
Mass of aluminium atoms in aluminium oxide $=2\times 27=54u$
Molecular mass of aluminium oxide $=2\times 27+3\times 16=54+48=102u$

Here; 2 mole aluminium ions are present in 102 g aluminium oxide
Hence, number of aluminium ions in 0.051 g aluminium oxide
$=\left(\frac{2}{102}\right)\times 0.051=\frac{2}{2000}$ mole
$=\frac{6.022\times {10}^{23}}{1000}=6.022\times {10}^{20}$

Why do we fall ill ? NCERT Solution

 Question 1: How many times did you fall ill in the last one year? What were the illnesses?

Answer: Last year, I suffered once from fever and twice from diarrhea.

1. Think of one change you could make in your habits in order to avoid any of/most of the above illnesses.
   
   Answer: I will stop buying food items from roadside vendor.
2. Think of one change you would wish for in your surroundings in order to avoid any of/most of the above illnesses.
   
   Answer: My neighborhood needs to be cleaner and drainage needs to be properly covered.

Question 2: A doctor/nurse/health-worker is exposed to more sick people than others in the community. Find out how she/he avoids getting sick herself/himself.

Answer: A doctor/nurse/health-worker takes good care of cleanliness. These people maintain a high level of personal hygiene. They also maintain cleanliness at their workplace. They usually wash hands after examining or attending to a patient.

Question 3: Conduct a survey in your neighbourhood to find out what the three most common diseases are. Suggest three steps that could be taken by your local authorities to bring down the incidence of these diseases.

Answer: Malaria, dengue and cholera are the most common diseases in my neighborhood.

Following steps should be taken by local authorities to bring down the incidence of these diseases:

• Drinking water supply should be improved in the nearby slum.
• Ditches should be filled and drains should be covered.
• There should be regular inspection to check for mosquito breeding.

Question 4: A baby is not able to tell her/his caretakers that she/he is sick. What would help us to find out

1. That the baby is sick?
   
   Answer: If the baby is crying too much and is restless, then baby’s caretakers get a clue about the baby being sick.
2. What is the sickness?
   
   Answer: A doctor is the best person to tell about the specific disease the baby may be suffering from.

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Question 5: Under which of the following conditions is a person most likely to fall sick?

1. When she is recovering from malaria.
2. When she has recovered from malaria and is taking care of someone suffering from chicken-pox.
3. When she is on a four-day fast after recovering from malaria and is taking care of someone suffering from chicken-pox. Why?

Answer: She is most likely to fall sick in the third condition, i.e. when she is on a four day fast after recovering from malaria and is taking care of someone suffering from chicken pox. This situation presents many risk factors. Her immunity is weak because she was suffering from malaria. She is not getting proper nutrition because of her four-day fast. Proximity to a patient of chicken pox presents further risk to her.

Question 6: Under which of the following conditions are you most likely to fall sick?

1. When you are taking examinations.
2. When you have travelled by bus and train for two days.
3. When your friend is suffering from measles. Why?

Answer: The third condition presents the biggest risk. Measles is an infectious disease. So, going near that person increases the chances of getting infected.