Chemistry Class 9 Chapter 1 Think-Thank, Federal board

 

Think-Thank

Chapter 1

Fundamentals of chemistry

 

 

1.       What mass of sodium metal contains same number of atoms as 12.00 g of carbon.

Ans.

12.00g of carbon = 1 mole of carbon

No. of atoms in 1 mole of C = N_A = 6.022 x 10²³

1 mole of Na = 23 g

So, no. of atoms in 1 mole of Na = N_A = 6.022 x 10²³

Hence, 23g of Na contains same number of atoms as 12g of C.

 

2.       What mass of oxygen contains same number of molecules as 42g of nitrogen.

Ans.

Molar mass of N₂ = 2 x 14 = 28g

28g of N₂ = 1 mole

1g of N₂ = 1 / 28 mole

42g of N₂ = (1/28) x 42

                 = 1.5 mole

No. of molecules = 1.5 x 6.022 x 10²³

                               = 9.03 x 10²³ molecules.

So, mass of 9.03 x 10²³ molecules of oxygen:-               

Molar mass of O₂ = 2 x 16 = 32g

                 Mass in grams = (No. of molecules x Molar mass) / N_A

                                            = ( 9.03 x 10²³ x 32 ) / 6.022 x 10²³

                                            = 47.98g = 48g

              Hence, 48g of oxygen has same no. of molecules as 42g of nitrogen.

3.       Calculate mass of one hydrogen atom in grams.

Ans.

1 mole of hydrogen = 6.022 x 10²³ atoms

6.022 x 10²³ atoms of hydrogen = 1.008g

1 atom of hydrogen = (1.008 / 6.022 x 10²³)g

                                    = 0.167 x 10^-23g

                                   = 1.67 x 10^-24g

Hence, 1 atom of hydrogen is of 1.67 x 10^-24grams.

4.       Observe the given figure. It shows particles in a sample of air.

a)       Count substances shown in the sample

There are five substances in the given sample.

b)       Is air a mixture or pure substance? Explain ?

Air is a homogenous mixture of various gases. Each gas does not do reaction but retain its identity.

c)        Identify the formula of each substance in air.

Nitrogen = N₂

Oxygen = O₂

Carbon dioxide = CO₂

Water vapours = H₂O

Argon = Ar

d)       Argue whether each substance in air is an element or a compound.

N₂ , O₂ and Ar are elements.

CO₂ and H₂O are compounds.

e)       What is the more common substance in air?

The most common substance in air is nitrogen. 78% of air is N₂.

5.       Calculate the number of hydrogen atoms present in 18g of H₂O.

Ans.

18g of H₂O = 1 mole of H₂O

1 mole of H₂O = 6.022 x 10²³ molecules

1 molecule of H₂O = 2-H atoms

6.022 x 10²³ molecules of H₂O = 2 x 6.022 x 10²³ H-atoms

                                                      = 12.44 x 10²³ atoms

                                                      = 1.24 x 10²³ atoms

Hence, 18g of H₂O has 1.24 x 10²³ atoms of H.

6.       Calculate total number of atoms present in 18g of H₂O.

Ans.

Total no. of atoms in 1 molecule of H₂O = 3 atoms ( 2-H , 1-O)

18g of H₂O = 1 mole = 6.022 x 10²³ molecules

6.022x 10²³ molecules of H₂O = 3 x 6.022 x 10²³

                                          = 18.06 x 10²³ atoms

                                          = 1.806 x 10²⁴ atoms.

                 Hence, 18g of H₂O has total 1.806 x 10²⁴ atoms.