Chemistry - Unit 3: Stoichiometry

what is the molecular formula of a compound?

the number and type of different atoms in one molecule

what is the empirical formula of a compound?

the simplest whole number ratio of the different atoms or ions in a compound

how to deduce the formula of a simple compound from the relative numbers of atoms or ions present in a model or a diagrammatic representation.

count the number of atoms of each element in the model/diagram and repeat for each element

how to deduce the formula of an ionic compound from the charges on the ions

• write out the symbols of each element and write their valencies on top

• to determine the valency of an element, refer to their group number

• swap the valencies of each element and write the number as subscripts (put the number below the element)

• use empirical formula when writing ionic compounds

what are the state symbols used in chemical equations?

• (g) - gas

• (aq) - aqueous

• (l) - liquid

• (s) - solid

how to construct symbol equations with state symbols

1. find the symbols and formulae for each element/compound

2. put into format and see if the number of atoms of each element are the same on either side of the arrow

3. add in state symbols

how to construct ionic equations

1. split up (aq) compounds into its ions

2. cancel out any ions that are the same on either side (a.k.a spectator ions)

3. add state symbols in

what is relative atomic mass (A_r)?

the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C

what is relative molecular mass (M_r)?

the sum of the relative atomic masses

what term is used for ionic compounds?

relative formula mass (M_r)

what is the mole?

the unit of amount of substance

what unit is used to measure moles?

mol

how many particles are in one mole/Avogadro constant

6.02×10²³

what formula is used to calculate moles?

mol = mass ÷ molar mass

what formula is used to calculate mass?

mass = molar mass × mol

what formula is used to calculate molar mass?

molar mass = mass ÷ mol

what formula is used to calculate the number of particles, using the Avogadro constant

mol × Avogadro constant

what formula is used to calculate volume of gas at r.t.p

volume of gas at r.t.p = mol × 24dm³/24000cm³

what 2 units can be used to measure conentration

1. g/dm³

2. mol/dm³

what formula can be used to calculate concentration?

concentration = mol ÷ volume

how to convert between cm³ and dm³?

cm³ → dm³ : ÷1000

dm³ → cm³ : ×1000

how to convert between g/dm³ and mol/dm³?

g/dm³ → mol/dm³: multiply by M_r

mol/dm³ → g/dm³: divide by M_r

how to calculate the empirical formula?

1. write the relative atomic masses of each element

2. divide the percentage masses by the relative atomic mass of each element

3. divide the answers by the smallest number from step 2 to find the ratio of atoms of each element

how to calculate molecular formula?

1. find the M_r of the empirical formula

2. divide the M_r of the molecular formula by the M_r of the empirical formula

3. multiply each in the empirical formula by the answer from step 2

how to calculate percentage yield?

percentage yield = (actual amount of product produced ÷ theoretical amount of product possible) × 100

how to calculate percentage mass?

percentage mass = (total A_r of element ÷ M_r of compound) × 100

how to calculate percentage purity?

percentage purity = (mass of pure substance ÷ total mass of sample) × 100