atoms, molecules, & stoichiometry Flashcards
(22 cards)
(a) CONCEPT 1 (STOICHOMETRY)
define
1) relative atomic mass (Ar) of an element
2) relative isotopic mass
3) relative molecular mass
4) relative formula mass
1) .. of an element is the average mass of 1 atom of the element relative to 1/12th the mass of 1 atom of the carbon-12 isotope
2) .. is the mass of 1 atom of the isotope relative to 1/12th the mass of 1 atom of the carbon-12 isotope
3) .. of a substance is the average mass of 1 molecule of the substance relative to 1/12th the mass of 1 atom of the carbon-12 isotope
4) .. of a substance is the average mass of 1 formula unit of the subtstance relative to 1/12th the mass of 1 atom of the carbon-12 isotope
(b) CONCEPT 1 (STOICHOMETRY)
define mole
One mole of a substance contains exactly 6.02 x 10^23 elementary identities
(c) CONCEPT 1 (STOICHOMETRY)
a particular type of stainless steel has the following composition:
Fe-55 38%
Fe-57 32%
Fe-52 12%
Cr-52 18%
based on the information, what is the relative atomic mass of iron?
Ar of Fe = (55 x 38 + 57 x 32 + 52 x 12)/(38+32+12) = 55.3 (1dp)
(c) CONCEPT 1 (STOICHOMETRY)
copper exists in 2 isotopic forms, Cu-63 and Cu-65.
given Ar of Cu = 63.5, what is the relative abundance of Cu-63?
let relative abundance of Cu-63 be x%
(63x + 65(100-x))/100 = 63.5
x = 75
know that copper only has 2 isotopic forms, so total relative abundance is 100%
(d) CONCEPT 1 (STOICHOMETRY)
define empirical and molecular formula
empirical formula is the simplest whole number ratio of the elements present in 1 molecule or formula unit of the compound.
molecular formula is the actual number of each of the different atoms present in the molecule.
(e) CONCEPT 1 (STOICHOMETRY)
what are the steps for finding empirical and molecular formula?
- elements
- mass per 100g
- amt/mol (mass/Ar)
- simpliest ratio
- let molecular formula of X be n(____), where n is an integer to be found
- assume mass per 100g
- working to determine molecular formula must still be shown even if n = 1
(g) CONCEPT 1 (STOICHOMETRY)
list all stoichiometry formulae
n = L/no. of particles
n = mass(g)/molar mass(gmol-1)
n = volume(dm3)/molar volume(dm3) (gas only, where volume ratio = gas ratio at same temp and pressure)
n = concentration(moldm-3) x volume(dm3)
(g) CONCEPT 1 (STOICHOMETRY)
what remains constant and what is changed during dilution and sampling?
dilution: mole remains constant, volume increases, concentration decreases
sampling: mole decreases, volume decreases, concentration remains constant
(g) CONCEPT 1 (STOICHOMETRY)
what is the formula relating before and after dilution?
number of moles before and after is the same
n1 = n2
c1v1 = c2v2
(g) CONCEPT 1 (STOICHOMETRY)
what is the trick to answering limiting reactant questions?
- given data for both reactants
- assume 1 is fully reacted, calculate mole ratio and match with given mole ratio
(g) CONCEPT 1 (STOICHOMETRY)
what is the trick to handling ratios?
the quickest way to get to 1 is to divide a number by itself
(h) CONCEPT 2 (TITRATION & COMBUSTION DATA)
what is the key to solving combustion data questions?
mole ratio = volume ratio
(a) CONCEPT 1 (STOICHOMETRY)
why is relative atomic mass of an element often not a whole number?
this arises due to the presence of isotopes occuring in different percentage abundance.
(i) CONCEPT 3 (REDOX)
what is conserved in redox reactions?
as oxidation and reduction have to occur simultaneously in redox reactions,
1. total number of electrons lost by RA = total number of electrons gained by OA (conservation of charge)
2. total increase in O.S. of RA = total decrease in O.S. of OA
(i) CONCEPT 3 (REDOX)
what is the exception to H having an oxidation number of +1?
-1 in metal hydrides (e.g. NaH, MgH2)
(i) CONCEPT 3 (REDOX)
what is the exception to oxygen having an oxidation state of -2?
-1 in peroxides (e.g. H2O2, BaO2)
-1/2 in superoxides (e.g. KO2)
(i) CONCEPT 3 (REDOX)
a reaction in which an element is simultaneously reduced and oxidised is known as:
a disproportionation reaction
Cu2O (s) -> CuO (s) + Cu (s)
(i) CONCEPT 3 (REDOX)
a reaction where 2 reactants, each containing the same element but with a different oxidation number, is known as:
a comproportionation reaction
Fe (s) + 2Fe3+ (aq) -> 3Fe2+ (aq)
(j) CONCEPT 3 (REDOX)
explain half-equation method for balancing redox equations
- KOHE
K: Balance the atoms of Key elements (other than H and O)
O: Balance Oxygen by adding H2O
H: Balance Hydrogen by adding H+
E: Balance charges by adding Electrons - Combine equations by balancing charges (conservation of charge) then cancel any repeated species
- (under basic conditions) Balance H+ by adding OH- to both sides of equation, combine H+ and OH- to form H2O, cancel out any extra H2O
(j) CONCEPT 3 (REDOX)
explain oxidation number method for balancing redox equations
- KOSOH
K: Balance the atoms of key elements involved in oxidation and reduction and write down individual O.S.
OS: Balance the CHANGE in TOTAL oxidation states
O: Balance Oxygen by adding H2O
H: Balance Hydrogen by adding H+ - (under basic conditions) Balance H+ by adding OH- to both sides of equation, combine H+ and OH- to form H2O, cancel out any extra H2O
(j) CONCEPT 3 (REDOX)
remember to revise redox titrations (pg 28 of notes)
plz do it
(j) CONCEPT 3 (REDOX)
what is the trick to determining unknown oxidation numbers through half-equation method?
conservation of charge (number of moles of electrons is conserved)
