Stoichiometry

Question 1

Construct equations with state symbols, including ionic equations

Answer 1

-Start​ ​with​ ​the​ ​balanced​ ​chemical​ ​equation
HCl(aq)​ ​+​ ​NaOH(aq)​ ​→​ ​NaCl​ ​(aq)​ ​+​ ​H2O​ ​(l)
-Split​ ​up​ ​(aq)​ ​compounds​ ​into​ ​its​ ​ions
H​+​​ ​+​ ​Cl-​ ​​ ​+​ ​Na+​ ​​ ​+​ ​OH​-​​ ​→​ ​Na​+​​ ​+​ ​Cl-​ ​ +​​ ​ ​H​2O​
-Cancel​ ​out​ ​the​ ​ions​ ​that​ ​are​ ​the​ ​same​ ​on​ ​either​ ​side​ ​–​ ​known​ ​as ‘spectator​ ​ions’,​ ​leaving​ ​the​ ​ionic​ ​equation
H​+​​ ​+​ ​OH​-​​ ​→​ ​H2​O

Question 2

Relative atomic mass, Ar

Answer 2

Average mass of naturally occurring atoms of an element on a scale where the 12C atom has a mass of exactly 12 units

Question 3

Relative molecular mass, Mr

Answer 3

Sum of the relative atomic masses. (Relative formula mass or Mr will be used for ionic compounds.)

Question 4

Mole and the Avogadro constant

Answer 4

• Mole​ ​=​ ​amount​ ​of​ ​substance
• The​ ​number​ ​of​ ​atoms,​ ​molecules​ ​or​ ​ions​ ​in​ ​a​ ​mole​ ​of​ ​a​ ​given​ ​substance​ ​is​ ​the Avogadro​​ constant: ​​6.02​​x​​102​3​​​per​​mole.

Question 5

Molar gas volume, taken as 24 dm3 at

room temperature and pressure

Answer 5

Volume​ ​(dm3​ )​ ​ ​of​ ​gas​ ​at​ ​RTP​ ​=​ ​Mol.​ ​x​ ​24

Question 6

Calculate stoichiometric reacting masses, volumes of gases and solutions, and concentrations of solutions expressed in g / dm3 and mol / dm3

Answer 6

moles​ ​=​ ​mass​ ​÷​ ​formula​ ​mass
● You​ ​can​ ​work​ ​out​ ​the​ ​moles​ ​or​ ​volume​ ​of​ ​a​ ​gas​ ​at​ ​RTP​ ​using​ ​the equation: volume​ ​=​ ​moles​ ​x​ ​24, rearranged​ ​to:​ ​moles​ ​=​ ​volume​ ​÷​ ​24
● You​ ​can​ ​work​ ​out​ ​concentrations​ ​of​ ​solutions​ ​in​ ​g/dm​3​​ ​using​ ​the​ ​equation: concentration​ ​=​ ​mass​ ​of​ ​solute​ ​÷​ ​volume
● you​ ​can​ ​work​ ​out​ ​concentrations​ ​of​ ​solutions​ ​in​ ​mol/dm3​ ​ using​ ​the​ ​equation: concentration​ ​=​ ​moles​ ​of​ ​solute​ ​÷​ ​volume
● to​ ​convert​ ​between​ ​concentration​ ​in​ ​mol/dm3​ ​​ ​and​ ​g/dm3:​ mol/dm3​ ​​ ​→​ ​g/dm​3​​ ​multiply​ ​by​ ​Mr, g/dm3​ ​​ ​→​ ​mol/dm3​ ​​ ​divide​ ​by​ ​Mr
● if​ ​you​ ​are​ ​given​ ​a​ ​reacting​ ​mass/volume/concentration​ ​and​ ​are​ ​asked​ ​to​ ​work out​ ​the​ ​mass/volume/concentration​ ​of​ ​another​ ​reactant​ ​or​ ​a​ ​product:
o calculate​ ​the​ ​number​ ​of​ ​moles​ ​(use​ ​the​ ​appropriate​ ​equation​ ​from
above)
● limiting​ ​reagents:
o in​ ​a​ ​reaction,​ ​often​ ​one​ ​of​ ​the​ ​reactants​ ​will​ ​be​ ​a​ ​limiting​ ​reagent.​ ​This means​ ​that​ ​this​ ​reactant​ ​will​ ​be​ ​used​ ​up​ ​first​ ​and​ ​will​ ​cause​ ​the​ ​reaction
to​ ​stop.
- ​if​ ​given​ ​the​ ​mass/volume​ ​of​ ​a​ ​limiting​ ​reagent​ ​and
another​ ​reagent,​ ​​you​ ​must​ ​use​ ​the​ ​mass/volume​ ​of​ ​the​ ​limiting​ ​reagent​.

Question 7

Empirical formulae and molecular formulae

Answer 7

-if​ ​there​ ​is​ ​a​ ​common​ ​multiple​ ​in​ ​the​ ​number​ ​of​ ​different​ ​elements​ ​in​ ​your molecular​ ​formula,​ ​then​ ​divide​ ​by​ ​this​ ​number​ ​to​ ​give​ ​you​ ​the​ ​simplest whole​ ​number​ ​ratio. e.g.​ ​Fe2​ O​ ​4,​ ​ ​common​ ​multiple​ ​is​ ​2,​ ​empirical​ ​formula​ ​is​ ​FeO2​
-Find​ ​relative​ ​molecular​ ​mass​ ​of​ ​the​ ​empirical​ ​formula, Divide​ ​relative​ ​molecular​ ​mass​ ​of​ ​compound​ ​by​ ​that​ ​of​ ​the​ ​empirical formula, If​ ​answer​ ​was​ ​2​ ​and​ ​the​ ​empirical​ ​formula​ ​was​ ​Fe2​ O​ ​3​​ ​then​ ​the​ ​molecular
formula​ ​would​ ​be​ ​empirical​ ​formula​ ​x​ ​2​ ​=​ ​Fe4​ O​ ​6

Question 8

Percentage yield and percentage purity

Answer 8

Percentage ​​yield​​= ​​​​​​​​​​​​​​​​​Amount ​​of​​ product​​ produced(AY)/ Maximum amount possible (TY) x 100
Percentage purity = Mass of impure/ mass of pure x 100