Question

The largest number of molecules is in

• A. 34 g of $H_2O$
• B. 28 g of $CO_2$
• C. 46 g of ethyl alcohol
• D. 54 g of nitrogen pentaoxide $(N_2O_ 5)$

Answer 1

The Correct Option is A

34 g of $H_2O$
$ \because $ 18 g of $H_2O = 6 \times 10^{23}$ molecules
$\therefore$ 34 g of $H_2O = \frac{6 \times 10^{23} \times 34}{18} $
= $11.33 \times 10^{23} $ molecules
(b) 28 g of $CO_2$
$ \because $ 44 g of $CO_2 = 6 \times 10^{23}$
$\therefore$ 28 g of $CO_2 = \frac{6 \times 10^{23} \times 28}{44} $
= $3.8 \times 10^{23} $ molecules
(c) 46 g of $CH_3OH$
$ \because $ 32 g of $CH_3OH = 6 \times 10^{23}$ molecules
$\therefore$ 46 g of $CH_3OH = \frac{6 \times 10^{23} \times 46}{32} $
= $8.625 \times 10^{23} $ molecules
(d) 54 g of $N _2O_5$
$ \because $ 108 g of $N _2O_5 = 6 \times 10^{23}$ molecules
$\therefore$ 54 g of $N _2O_5 = \frac{6 \times 10^{23} \times 54}{108} $
= $3 \times 10^{23} $ molecules

Answer 2

Ans: One mole of a substance is the amount of substance that has the same number of particles as the number of carbon atoms in 12g of C-12 carbon. In 1 mole of a substance there are 6.023 x 10²³ number of atoms.

To calculate the number of molecules in a substance, find the molecular weight of the substance. This molecular weight will have the avogadro number of molecules. Now calculate the molecules for the given mass of the substance. 

In the question: 

1. 34g of H₂O

Molecular weight of H₂O is 18g 

18g of H₂O = 6.023 x 10²³ molecules 

Now, 34g of H₂O  = 6.023 x 10²³ x 34/18 

= 11.33 x 10²³ molecules 

1. 28 g of CO₂

Molecular weight of CO₂ is 44 g 

44 g of CO₂ =  6.023 x 10²³ molecules 

Now, 28g of CO₂ = 6.023 x 10²³ x 28/44

=3.8 x 10²³ molecules 

1. 46 g of ethyl alcohol

Molecular weight of ethyl alcohol = 32g 

32 g of ethyl alcohol = 6.023 x 10²³ molecules 

Now, 46 g of ethyl alcohol  = 6.023 x 10²³ x 46/32

= 8.625 x10²³ molecules 

1. 56g of N₂O₅

Molecular weight of N₂O₅ = 108g 

108 g of N₂O₅ = 6.023 x 10²³ molecules 

Now, 56 g of N₂O₅ = 6.023 x 10²³ x 56/108

= 3 x 10²³ molecules 

Therefore, 34g of water has the highest number of molecules.