Chemistry - Basic Calculations
EquationsChemical Equation
This is a word equation:
magnesium + oxygen ➡️ magnesium oxide
However, it can't tell the physical states of the substances and the relative number of particles, so chemists usually use chemical equations:
Chemical equation for magnesium burning in air
To let chemical equations show the correct relative number of particles, we have to balance the equations after writing the reactants and products down.
Balancing an equation is to make sure the numbers of the same atoms are the same on both side of the equation
Displacement Reactions and Ionic Equations
When we place a metal in solution of a less reactive metal, the more reactive metal will displace the less reactive metal from the solution. This type of reaction is called displacement reaction.
Displacement of copper and silver
We can rewrite the chemical equation, showing the mobile ions represent:
Turn the molecules into ions form
We can find that some of the ions don't change after the reaction, that is, they are not involved. We erase them then.
Ionic equation formed
This equation is called ionic equation.
Mole and Other Things About ItMole
A special unit used to describe the quantity of particles in a substance. Numerally the same to stoichiometric coefficients.
Avogadro Constant
Chemists have chosen the number of atoms in exactly 12.0g of carbon-12 as the reference unit for the mole, it is 6.02*10^(23). Thus, one mole of any substance contains 6.02*10^(23) formula units.
Formula Mass and Molar Mass
One mole of a substance has a mass equal to its formula mass expressed in gram unit. The mass of one mole of a substance is called its molar mass（g/mol).
Molar masses of elements can be checked in the periodic table (same as the atomic masses), and molar masses of molecules can be calculated by adding up the molar masses of relative elements in it. Similarly, we can calculate the percentage (element mass / compound mass * 100%) and mass (using mass and percentage given) of an element in a compound.
Number of Moles
Number if moles (mol) = mass (g) / molar mass (g/mol) = number of formula units / Avogadro constant (1/mol)
Chemical Formulae of CompoundsEmpirical Formula
Formula which shows the simplest whole number ratio of the atoms or ions present. Applicable to all compounds.
Molecular Formula
Formula which shows the actual number of each kind of atoms in one molecule of the substance. Only applicable to molecular compounds and elements consisting of molecules.
Structural Formula
Formula which shows how the constituent atoms are joined up in one substance.
Formulae of ethane
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