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Question

Which of the following gas is lighter than air?


A

Oxygen

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B

Ammonia

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C

Chlorine

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D

Carbon Dioxide

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Solution

The correct option is B

Ammonia


The explanation for the correct option:

Option (B) Ammonia

  • To check for the gas lighter than air, calculate the vapor density of all these gases and compare them with the vapor density of air.

Vapor density

The weight of a vapor or gas compared to an equal volume of air is known as vapor density.

Step 1: Given data

The following gases are given: Oxygen, Ammonia, Chlorine, and Carbon dioxide.

The vapor density of air is 14.4.

Step 2: Formula used

The vapor density can be calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Step 3: Calculation of vapor density of ammonia

The vapor density of ammonia is calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Mass of 1 mole of ammonia = 17g

vapourdensityofammonia=172=8.5

Hence, ammonia's vapor density is less than air's vapor density. Therefore ammonia is lighter than air.

The explanation for incorrect options:

Option A: Oxygen

Step 1: Given data

The following gases are given: Oxygen, Ammonia, Chlorine, and Carbon dioxide.

The vapor density of air is 14.4.

Step 2: Formula used

The vapor density can be calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Step 3: Calculation of vapor density of oxygen

The vapor density of oxygen is calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Mass of 1 mole of oxygen= 32g

vapourdensityofoxygen=322=16

As the vapor density of oxygen is more than that of air, it is heavier than air.

Hence, this option is incorrect.

Option C: Chlorine

Step 1: Given data

The following gases are given: Oxygen, Ammonia, Chlorine, and Carbon dioxide.

The vapor density of air is 14.4.

Step 2: Formula used

The vapor density can be calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Step 3: Calculation of vapor density of chlorine

The vapor density of chlorine is calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Mass of 1 mole of chlorine = 70g

vapourdensityofchlorine=702=35.5

Hence, this option is incorrect.

Option D: Carbon dioxide

Step 1: Given data

The following gases are given: Oxygen, Ammonia, Chlorine, and Carbon dioxide.

The vapor density of air is 14.4.

Step 2: Formula used

The vapor density can be calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Step 3: Calculation of vapor density of carbon dioxide

The vapor density of dioxide is calculated using the formula: vapourdensity=massof1moleofgasmassof1moleofhydrogen

Mass of 1 mole of carbon dioxide = 44g

vapourdensityofcarbondioxide=442=22

Hence, this option is incorrect.

Hence, option B, ammonia is the correct option.


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