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# Density

## Density Formula and Measurement

Density formula of solid, liquid, and gas molecules or chemical elements is the measurement of mass per unit volume of material at a given temperature and pressure. A Standard or normal density calculator uses for calculating densities at 0°C and one atm pressure with CGS and SI units gm/cm3 and kg/m3 in chemistry or physics. Therefore, the gas density is the calculation of the ratio of the equal volume masses of gas and oxygen. The atomic mass of oxygen taken 16 gm/atom. The density of the gas also related to the diffusion by Graham’s law. The ideal gas equation uses to calculate the molecular weight and density of ideal gases but this equation approximately obeyed under normal conditions. Therefore, the molar mass measurement by the ideal gas equation, not an accurate one.

### Limiting Density of Gases

The ideal gas law for n-mole gases, PV = nRT, or, P = nRT/V = gRT/MV. ρ = symbol of density measurement of the gas, M = mass (gm) per unit volume (cc), called normal density formula per unit pressure. Therefore, the molar mass of the gas can be experimentally determined by using this expression. Such density definition or calculation from the above gas equation measured only an approximate and not an accurate one for learning chemistry or chemical science. But the above equation accurately obeyed at very low pressure at P→0. So the molar mass value of this pressure gives accurate and the equation represented as, M = (ρ/P)P→0 ×RT, where (ρ/P)P→0 = limiting the density of a gas.

But direct use of this relation poses some difficulty since, at P→0, ρ→0 and not possible to determine experimentally. Therefore, the above graphical measurement has taken advantage to calculate molar mass by density.

Generally, the value of the molar mass of gas measured from the density comparison with oxygen at the same temperature. For example, ρ of CH3F = 1.5177 while for oxygen = 1.4177. Thus, MCH3F = MO2 × (1.5177/1.4177) = 32 gm/mole, this is the correct molar mass of CH3F.

### Abnormal Vapour Density Calculator

Vapour density of a substance defined as the ratio of the density of gaseous state of the substance and hydrogen under the same temp and pressure. It is the quantity having no unit or dimensions and reprinted as, D0 = M0/2, where M0 = formula molecular weight of the gasses.

But for many chemical substances, like ammonium chloride, phosphorus pentachloride, hydrogen peroxide, etc has calculated vapour densities less than the theoretical measurement. Only at a high temp D0≈D0/2. This phenomenon is called abnormal vapour densities. The cause of this phenomenon due to the thermal bond dissociation of the above chemical substances.

NH4Cl → NH3 + HCl
PCl5 → PCl3 + Cl2
N2O4 → 2NO2
I2 → 2I

### Formula for Calculating Abnormal Density

At constant pressure due to the dissociation of the chemical bonds, the volume of the gases increases with the increase of the mole number. Since mass remains the same and D0 decreases. The extent or fraction of the total number of molecules that suffer dissociation calculate the degree of dissociation. Let us take one molecule of substance A splits up into n molecules of B by the specific heat, A → nB.

At equilibrium for each gm mole of A will be (1 – α) gm moles of undissociated A and nα gm moles of B. So the total number of gm mole of gas present = (1-α) + nα or 1 + (n-1)α. When V = volume occupied per unit volume of gas or vapour and D0 define the density measurement absence of any dissociation. If D denotes the observed density when actual dissociation occurred and the total weight of the gas = W.

W = D0V and W = D×V[1 + (n-1)α]
∴ D0/D = 1 + (n-1)α

This expression shows that, at high temperature, α = 1 (complete bond dissociation) and for NH4Cl, and PCl5, n=2, thus D = D0/2, and this density formula in chemical science also saw in the experimental calculator.