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Density- Types of Density, Definitions, and Formulas

Have you ever wondered why a ship floats on water but a nail sinks? Seems a bit weird, does it not? The heavier object (in this case ship) floats but the lighter object (in this case nail) sinks.

Lets us try a similar routine for chemical compounds. What will happen if we try to add oil to the water?

Most of you know already know the answer that such a mixture will not mix and remain as separate layers. What causes such behaviour? 

One of the parameters which control miscibility is density. Let us take a closer look at it now.  

image                           

Figure of Oil in Water

Table of Contents

Types of Density

Generally, we study two types of density

  • Absolute Density
  • Relative Density

Absolute Density

Absolute density (d) is defined as the mass of any material per unit of volume. Generally, expressed in g cm-3.

IMAGE

For gas, Absolute density (d) is defined as the molar mass of any gas per unit of molar volume of gas. 

IMAGE

Units of Density

Density expressed in g cm-3 or g mL-1

S.I unit of density is Kg m-3

Relative Density

Relative density is defined as the density of a substance with respect to any other substance.

 Specific gravity: Specific gravity is defined as the density of any substance divided by the density of water at 4oC (generally used for solids and liquids). 

IMAGE (Specific gravity is unitless quantity)

Density of water at 4oC is 1 g cm-3

For gas:

Vapour Density

Vapour density is defined as the density of the gas with respect to hydrogen gas at same temperature and pressure.

Vapour densityIMAGE(at the same Temperature & Pressure)

Vapour density IMAGE (at same Temperature & Pressure)IMAGE

We know, according to Avogadro’s hypothesis, All gases having equal volume have the same number of gaseous molecules (not atoms) at same temperature and pressure.

[So, at the same Temperature and Pressure, the molar volume of gas = molar volume of H2 gas ]

 

Vapour densityIMAGE(at same Temperature & Pressure)

 

 Vapour density IMAGE

Practice Problems on Density

Question1. The specific gravity of a solution is 2.1, then calculate the mass of 180 ML of the same solution.

  1. A. 210 g
  2. B. 389 g
  3. C. 378 g
  4. D. 350 g

Answer: (C)

Solution: Specific gravity is unitless quantity, IMAGE

Question2.  What is the relative density of NO2 gas with respect to CO2 gas at the same temperature and pressure?

  1. A. 1.50
  2. B. 1.045
  3. C. 2.045
  4. D. 1.90

Answer: (B)IMAGE

according to Avogadro’s hypothesis, All gases having equal volume have the same number of gaseous molecules (not atoms) at same temperature and pressure.

[So, at the same T and P molar volume of CO2 = molar volume of NO2 gas ]IMAGE

Question3.  Find vapour density of nitrogen gas is

  1. A. 7
  2. B. 14
  3. C. 28
  4. D. 32

Answer: (B)

Solution: Nitrogen gas in free state present in the form of N2.

IMAGE

Question4.  The vapour density of nitrogen gas is

  1. A. 7
  2. B. 14
  3. C. 28
  4. D. 32

Answer: (B)

Solution: Nitrogen gas in free state present in the form of N2.

IMAGE

FAQs on Density

Question1. Why density of water is higher than ice?

Answer: As we start heating ice, hydrogen bonds break and water molecules start occupying the empty space present in ice (ice has a cage-like structure).

Question2.  Does the density of any material depend on the quantity of matter?

Answer: No, Density does not depend on the quantity of matter. On reduction of the quantity of material, its mass and volume will change but there will not.

Question3. Density is additive in nature or not like mass and volume.

Answer: No, the Density of two substances can’t be added algebraically like mass and volume, because density is an intensive property. 

Question4.  Density is temperature-dependent or not?

Answer: Yes, density depends on temperature. At higher temperatures, the kinetic energy of constituent particles increases and density is reduced. 

Related Topics to Density

Strength Molality
Avagadro’s Hypothesis Parts per Million
Atomic Mass Definition of Equivalent Weight
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