# Chemistry Amount of a substance

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## Balancing equations

• Identify reactants and products
• reactant coefficients

Tips

• if unchanged leave out
• compounds then elements
• multiple for smallest common value
• coefficient is number of moles
• dont change formulae
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## Ionic Equations

positive ions are anions

negative ions are cations

compounds made of ions are salts and are neutral. they are gaint ionic lattice.

Group number represents charge. Metals are always positive and non-metals are always negative.

Roman numeral represents charge too.

ions are free to move in aqueous solutions. ions that are unchanged are spectator ions.

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## Relative atomic mass and relative molecular mass

Relative atomic mass (Ar) is the average mass of one atom of an element relative to 1/12 of the mass of one atom of carbon-12

Relative molecular mass (Mr) is the mass of one molcule of a covalent substance relative to 1/12 the mass of one atom of carbon-12

ion compounds form ionic lattice so relative formula mass is used.

Mass =Mr X moles

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## The mole and Avogadro Constant(L)

12g of carbon-12 = 1 mole of carbon-12

A mole is the amount of substance that contains as many particles as there are in the atoms of 12g of Carbon-12

Avogrado's constant is the number of particles in one mole of any substance.

Moles = mass/Ar or Mr

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## Associated Calculations

the amount of solute present in a fixed quantity of solvent or solution is called the concentratioof the solution.(moldm-3)

number of moles = concentration X volume of solvent

cm3 to dm3 is divide by 1000

gdm3 divide by Mr gives moldm-3

Titration

• known solution in the burette

Percentage yeild = actual mass/ theoretical mass X 100

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## Empirical and Molecular Formula

the empirical formula of a compound is the forumla which shows the simplest whole-number ratio of atoms in a compound.

the molecular formula shows the number of each type atom.

C            H

moles ratio        85.8/12    14.2/1     Mass/Ar

value of moles     7.15            14.2   divide by the smallest value

empirical formula   1       :          2  CH2

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## Atom economy

the atom economy of a reaction is the percentage of the total mass of the reactants that can in theory convert into the desired product.

Atom economy = Molecular mass of the desired product/ sum of molecular masses of all reactants X 100

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## the ideal gas equation

• the temperature - hotter fast particles more space
• the pressure - higher more compressed and less space
• the amount of gas - more gas more space

pV=nRT  p = pressure Pa

V= volume M3

n = number of moles

R = gas constant: 8.31 Jmol-1K-1

T = temperature K

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## the ideal gas equation

• the temperature - hotter fast particles more space
• the pressure - higher more compressed and less space
• the amount of gas - more gas more space

pV=nRT  p = pressure Pa

V= volume M3

n = number of moles

R = gas constant: 8.31 Jmol-1K-1

T = temperature K

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