Chapter 11 stoichiometry

Calculating the percentage of an element in a compound (by mass)

(Mass of element's contribution to compound/mass of whole compound) x 100

Molecular formula

Is the actual number of the different elements in a compound

Empirical formula

Is the simplest whole number ratio of the different elements in a compound

Comparing ratios in empirical formula

You can only compare ratios of elements in moles

Finding empirical formula from percentage of elements given

%→ (=) grams →(÷Mr) moles → simplest whole number

When given percentages we can assume we have 100g of the compound starting off

Finding empirical formula from reaction given

When given reactions, we need to work out the mass of each part of the compound involved in the empirical formula (usually involves subtraction)

Hydrated compounds

Have a molecule of water attached (.xH2O)

Anhydrous compounds

Do not have molecules of water attached

Heating a hydrated compound

Will remove the water and turn it anhydrous

Working with balanced equations

You must work in moles when working with balanced equations

Words that refer to left-hand side of the equation

Reactants/reacts/reacted/required/combusts(in the case of a reaction with oxygen)

Words that refer to right-hand side of the equation

Products/produced/formed/released(in the case of a gas)/ precipitated (in the case of a solid)

The balanced equation given in the question is

Hypothetical, it does not show how many moles you actually have

More than one balanced equation

(the product of one reaction becomes the reactant of the new reaction)

The number of moles is "carried down“ and is also the same number of moles when it is the reactant

The molar ratio is NEVER involved when bringing a number of moles from one equation to another