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×Graham's Law of Diffusion and Effusion

Graham's law of diffusion (or Graham's law of effusion) is a law that expresses the relationship between the rate of diffusion or effusion to molar masses of particles. This empirical law was stated by Scottish chemist Thomas Graham in 1848. He established the relationship through experiments.

Dalton's Law of Partial Pressures

Dalton's law is also known as the law of partial pressure or Gibbs-Dalton law (rarely). The law describes the relationship between the total pressure of a mixture of non-reacting ideal gases and the partial pressures of each individual component. Dalton's law is valid for ideal gases. The law is similar to Amagat's law of additive volumes.

An amagat is a unit of number density. It is similar to Loschmidt constant which is the molecular density quoted at STP (*T* = 273.15 K, *P* = 1 atm). 1 amagat is defined as the number of molecules of an ideal gas per unit volume at STP. 1 amagat is equivalent to Loschmidt constant. The symbol used for amagat is amg or Am (rarely). It is a non-SI unit.

The number is named after Émile Hilaire Amagat. He was a French physicist and also known for Amagat’s law of additive volume.

Amagat's Law of Additive Volumes

Amagat's law of additive volumes is the law of partial volumes. The law relates the total volume of a mixture with the volumes of individual components. Amagat's law is very similar to Dalton's law of partial pressure. The law is only valid for ideal gases. The law is named after Emile Amagat who was a French Physicist. He published his law of partial volumes in 1880.

The Loschmidt constant is also called as the Loschmidt number. The symbol used for the Loschmidt constant is *n*_{0}. It is the number of molecules of an ideal gas per unit volume. The Loschmidt constant has the unit of the reciprocal cubic metre. At STP (*P* = 1 atm, *T* = 273.15 K), The value of the constant is 2.686 781 1 × 10^{25} m^{−3}. The constant is a measure of number density.

The Avogadro constant or (the Avogadro number earlier) is the number of elementary units in one mole of any substance. The Avogadro constant is denoted as *N*_{A}. It has the dimension of the reciprocal amount of substance (mol^{−1}). The approximate value of *N*_{A} is 6.022 × 10^{23} mol^{−1}. This means one mole of any substance contains 6.022 × 10^{23} elementary particles. The Avogadro constant is named after Italian scientist Amedeo Avogadro.

Accuracy and Precision in Chemistry

Accuracy refers to the closeness between the measured value and actual value of a quantity. While precision indicates closeness among the measured values of a quantity. Accuracy indicates how much the measured value agrees with actual value, and precision indicates how close the measured values agree among themselves. The results which are easily reproducible have good precision.

Significant Figures in Chemistry

In chemistry, we may deal with different varieties of calculations. Sometimes, we may come across very large numbers like Avogadro’s constant (*N*_{A} = 6.022 × 10^{23} mol^{−1}), or sometimes, with very small numbers like the diameter of hydrogen (120 pm). Many times, chemistry experiments involve the handling of large experimental data like vapour pressure data. So, there is a need for the right technique to manage numbers conveniently and maintain the accuracy of numbers. This is where the concept of significant figures arises.

SI Units Rules and Style Conventions

To maintain consistency in SI units, the International System of Units has adopted specific rules and style conventions for writing SI units. General rules for writing SI units were first specified by the 9th CGPM in 1948. Later, various international bodies like ISO extended these rules. These rules are very important in scientific writing. Adherence to these rules is strictly recommended while submitting and reviewing a manuscript. The general guidelines are described below.

The SI stands for the International System of Units or Le Systeme International d’Unités (in French). It is the modern system of measurement. The article lists some of them.

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