CBSE CHEMISTRY CLASS 9TH CHAPTER - 3 ATOMSAND MOLECULES

1. Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. Molar atomic mass of magnesium is 24g/ mol

2. Verify by calculating that

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9th Mole concept numerical problems solved-CBSE SET- 1

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2. Verify by calculating that

(a) 5 moles of CO

^{2}and 5 moles of H_{2}O do not have the same mass.
(b) 240 g of calcium and 240 g magnesium elements have a mole ratio of 3:5.

**3.**Find the ratio by mass of the combining elements in the following compounds.

(a) CaCO

_{3}(b) MgCl_{2}(c) H2SO4 (d) C2H_{5}OH (e) NH_{3}(f) Ca(OH)_{2}**4.**Calcium chloride when dissolved in water dissociates into its ions according to the following equation.

CaCl

_{2}(aq) → Ca_{2}+ (aq) + 2Cl– (aq)
Calculate the number of ions obtained from CaCl

_{2}when 222 g of it is dissolved in water.**5.**The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.

**6.**Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in 225 g of pure HgS? Molar mass of Hg and S are 200.6 g /mol and 32 g/ mol respectively.

**7.**The mass of one steel screw is 4.11g. Find the mass of one mole of these steel screws. Compare this value with the mass of the Earth (5.98 × 10

^{24}kg). Which one of the two is heavier and by how many times?

**8.**A sample of vitamin C is known to contain 2.58 ×10

^{24}oxygen atoms. How many moles of oxygen atoms are present in the sample?

9. Compute the difference in masses of 103 moles each of magnesium atoms and magnesium ions. (Mass of an electron = 9.1×10

^{–31}kg)
10. Compute the number of ions present in 5.85 g of sodium chloride.

**11.**A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?

12. Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is 9.1×10

^{–28}g). Which one is heavier?**13.**A silver ornament of mass ‘m’ gram is polished with gold equivalent to 1% of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.

**14.**A sample of ethane (C2H6) gas has the same mass as 1.5 ×10

^{20}molecules of methane (CH

_{4}). How many C

_{2}H

_{6}molecules does the sample of gas contain?

**15.**Fill in the blanks

(a) In a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is called ————.

(b) A group of atoms carrying a fixed charge on them is called ————.

(c) The formula unit mass of Ca

_{3}(PO_{4})_{2}is ————.
(d) Formula of sodium carbonate is ———— and that of ammonium sulphate is ————.

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9th Atoms and Molecules -

*Problems based on mole concept*
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