Hint: Apply the Law of Conservation of Mass to determine the mass of elements in a mixture.

Question.1. A student wants to make a homogeneous mixture of salt, sugar, and water that weighs 300 g. The student has 50 g sugar and 70 g salt. How much water should he add to the mixture?

(a) 70 g
(b) 120 g
(c) 150 g
(d) 180 g

Question.2. A student makes two solutions using components as listed in the table.cbe-questions-science-class-9-ch-3-q-2The student notices that sand settles at the bottom of the beaker in Solution 1. Which solution would be heavier?

(a) Solution 1, because sand is heavy.
(b) Solution 2, because it has more water.
(c) Solution 1, because it has more salt that disappears in water.
(d) Solution 2, because the total mass of components is greater.

Ans.1. (d) 180 g
Ans.2. (d) Solution 2, because the total mass of components is greater.

Hint: Apply the Law of Constant Proportions to estimate the amount of elements required in a chemical substance and identify postulates of Dalton’s atomic theory.

Question.3. Which option supports Dalton’s atomic theory?

(a) Atoms of an element have identical mass.
(b) Atoms can be created by chemical reactions.
(c) Atoms of different elements have similar chemical properties
(d) Atoms of an element change their properties in chemical reactions.

Question.4. A student has a sample of 200 g of table sugar (sucrose). For 200 g of sucrose, there are 84 g of carbon. Based on Dalton’s atomic theory, how much carbon will be there in 300 g of sucrose?
(Sucrose = C_{12}H_{22}O_{11})

(a) 42 g
(b) 23.8 g
(c) 56 g
(d) 126 g

Ans.3. (a) Atoms of an element have identical mass.
Ans.4. (d) 126 g

Hint: Correlate the fact of invisibility of atoms to the size of atoms.

Question.5. A student argues that atoms are invisible to the eyes. Which option supports the student’s argument?

(a) they exist freely
(b) they are in gaseous form
(c) they are very small in size
(d) they are in constant motion

Question.6. A student finds the atomic radii of some elements.cbe-questions-science-class-9-ch-3-q-6To see how an atom would appear, he observes an aluminum foil with a hand lens but is unable to see atoms that made up the foil. What can be the possible reason for this?

(a) Atoms of only gaseous substances can be observed.
(b) Atoms are very tiny and cannot be observed by a simple hand lens.
(c) Atoms are joined with great force, so it is difficult to distinguish one atom.
(d) Atoms of metals are in constant motion so they cannot be observed with a hand lens.

Ans.5. (c) they are very small in size
Ans.6. (b) Atoms are very tiny and cannot be observed by a simple hand lens.

Hint: List atomic symbols of commonly known elements as per IUPAC.

Question.7. Which symbol is a correct representation of cobalt according to IUPAC?

(a) CO
(b) Co
(c) co
(d) cO

Question.8. A student lists four compounds.
PCl_{5}, KBr, P_{4}O_{10}, Na_{2}CO_{3}
Which compound contains potassium?

(a) KBr
(b) PCl_{5}
(c) P_{4}O_{10}
(d) Na_{2}CO_{3}

Ans.7. (b) Co
Ans.8. (a) KBr

Hint: Recognize that different elements have different atomic mass.

Question.9. The table lists the mass of some atoms.cbe-questions-science-class-9-ch-3-q-9Which two atoms belong to the same element?

(a) P and Q
(b) R and S
(c) Q and R
(d) P and S

Question.10. The table lists atomic masses and atomic numbers of three elements.cbe-questions-science-class-9-ch-3-q-10A student studies that magnesium (Mg) and iron (Fe) react with oxygen (O) to form MgO and FeO. Although both compounds contain two atoms each of their molecular masses are different. What is the reason for the difference in their molecular masses?

(a) Difference in the atomic mass of iron and magnesium
(b) Difference in the number of iron and magnesium atoms
(c) Difference in the atomic number of iron and magnesium
(d) Difference in the number of O atoms in the two compounds

Ans.9. (d) P and S
Ans.10. (a) Difference in the atomic mass of iron and magnesium

Hint: Determine the number of atoms present in an element on the basis of their atomicity.

Question.11. The student lists some elements and their atomicity.cbe-questions-science-class-9-ch-3-q-11What is the number of atoms in each element?

(a) cbe-questions-science-class-9-ch-3-q-11-a(b) cbe-questions-science-class-9-ch-3-q-11-b(c) cbe-questions-science-class-9-ch-3-q-11-c(d) cbe-questions-science-class-9-ch-3-q-11-d

Question.12. The table shows the atomicity of different elements.cbe-questions-science-class-9-ch-3-q-12 Why is ozone labelled as triatomic?

(a) because ozone can exist freely in nature
(b) because ozone is a gas and gases are triatomic
(c) because three atoms of oxygen combine to form ozone
(d) because three molecules of oxygen combine to form ozone

Ans.11. (a) cbe-questions-science-class-9-ch-3-q-11-aAns.12. (c) because three atoms of oxygen combine to form ozone

Hint: Apply the law of constant proportions to calculate the mass ratio of atoms in a molecule.

Question.13. Which is the correct formula to calculate the mass ratio of ammonia (NH_{3})?

(a) \frac{mass  of  nitrogen}{(mass  of  hydrogen)^{3}}
(b) \frac{(mass  of  nitrogen)^{3}}{mass  of  hydrogen}
(c) \frac{mass  of  nitrogen}{3\times mass  of  hydrogen}
(d) \frac{3\times mass  of  nitrogen}{mass  of  hydrogen}

Question.14. The table lists some compounds and their mass ratio.cbe-questions-science-class-9-ch-3-q-14Which compound does NOT support the law of constant proportions?

(a) H_{2}O
(b) CO_{2}
(c) NO_{2}
(d) MgS

Ans.13. (c) \frac{mass  of  nitrogen}{3\times mass  of  hydrogen}
Ans.14. (b) CO_{2}

Hint: Write chemical formulae using symbols & valencies.

Question.15. A student learns that aluminum forms compound with chlorine and oxygen. She records the valencies of the three elements.cbe-questions-science-class-9-ch-3-q-15Which option gives the correct formula of aluminium oxide and aluminum chloride?

(a) aluminium oxide: Al_{2}O_{3}; aluminium chloride: AlCl_{3}
(b) aluminium oxide: 3AlO_{2}; aluminium chloride: 3AlCl_{2}
(c) aluminium oxide: Al_{3}O_{2}; aluminium chloride: Al_{3}Cl_{2}
(d) aluminium oxide: 3(AlO)_{2}</span>; aluminium chloride: 2(Al_{3}Cl)

Question.16. The table lists valencies of different elements.cbe-questions-science-class-9-ch-3-q-16What will be the chemical formula of sodium oxide?

(a) Na_{2}O
(b) NaO_{2}
(c) 2NaOH
(d) 2Na(OH)

Ans.15. (a) aluminium oxide: Al_{2}O_{3}; aluminium chloride: AlCl_{3}
Ans.16. (a) Na_{2}O

Hint: Identify ionic compounds whose formula unit mass can be calculated.

Question.17. The table lists some compounds.cbe-questions-science-class-9-ch-3-q-17Identify the compounds whose formula unit mass can be calculated.

(a) 1 and 3
(b) 2 and 3
(c) 2 and 4
(d) 1 and 2

Question.18. What is the formula unit mass of Na_{2}O?

(a) 20 u
(b) 39 u
(c) 55 u
(d) 62 u

Ans.17. (c) 2 and 4
Ans.18. (d) 62 u

Hint: Calculate the relative molecular mass of commonly known chemical compounds.

Question.19. Which option correctly represents the molecular mass of C_{6}H_{12}O_{6}?

(a) 29 u
(b) 43 u
(c) 96 u
(d) 180 u

Question.20. A student is studying Haber’s process to produce Ammonia. She learns that nitrogen and hydrogen react to form ammonia in the chemical reaction as shown.
N_{2}+3H_{2}\rightleftharpoons 2NH_{3}
Which option gives the ratio of nitrogen and hydrogen that participate in the reaction?

(a) 14:3
(b) 14:1
(c) 28:3
(d) 14:6

Ans.19. (d) 180 u
Ans.20. (a) 14:3

Hint: Calculate the number of moles, mass, atoms and molecules using Avogadro’s number.

Question.21. What will be the number of particles in 36 g of carbon atoms?

(a) 1.003 \times 10^{3}
(b) 2.008 \times 10^{3}
(c) 18.066 \times 10^{3}
(d) 36.132 \times 10^{3}

Question.22. The image shows four samples of elements.cbe-questions-science-class-9-ch-3-q-22 Which sample contains the greatest number of atoms?

(a) sample 1
(b) sample 2
(c) sample 3
(d) sample 4

Ans.21. (c) 18.066 \times 10^{3}
Ans.22. (d) sample 4

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