Chemical Calculations | General Chemistry 2
Mass-Mole-Number Relationship
Mole number n (in mol):
The number of moles in a sample
m = mass of the substance (in g)
M = molar mass of the substance (in g.mol-1)
N = number of particles in the substance
NA = Avogadro’s number = 6.022 x 1023 mol-1
Number of mole n in 2.0 g of N2:
n =
mN2 = 2.0 g
MN2 = 2 x MN = 2 x 14.0 = 28.0 g.mol-1⇒ n = = 7.1 x 10-2 mol
Number of nitrogen atom NN in 2.0 g of N2:n = =
mN2 = 2.0 g
MN2 = 28.0 g.mol-1
NN2 = 2 x NN
NA = 6.022 x 10-23 mol-1⇒ NN2 = NA x = 4.3 x 1022 atoms
NN = 2 x NN2 = 8.6 x 1022 atoms
Percent Composition by Mass
The percent composition by mass is the percent of the total mass contributed by each element of a compound. It is calculated as follows:
%X = n x x 100%
%X = percent composition of X
n = number of atoms X in a molecule of the compound
Mx = atomic mass of X (in amu or g.mol-1)
Mcompound = molecular mass of the compound (in amu or g.mol-1)
Determine %Al in Al2(SO4)3:
%Al = n x x 100%
n = 2 [2 atoms of Al in one molecule of Al2(SO4)3]
MAl = 26.98 g.mol-1
MAl2(SO4)3 = 2 MAl + 3 MS + 12 MO = 342.14 g.mol-1%Al = 2 x x 100 = 15.77 %
Stoichiometry
Stoichiometric coefficients:
The numeric values written to the left of each species in a chemical equation to balance the equation. The stoichiometric coefficients can be interpreted as the number of molecules or the number of moles of a substance produced or consumed during the reaction
3 H2 + N2 → 2 NH3
Molecular interpretation: 3 molecules of H2 react with 1 molecule of N2 to form 2 molecules of NH3
Molar interpretation: 3 moles of H2 react with 1 mole of N2 to form 2 moles of NH3
Stoichiometry:
The calculations of the masses, moles, or volumes of reactants and products involved in a chemical reaction. Reactants are said to be in stoichiometric amounts when their are combined in the same relative amounts as those represented in the balanced chemical equation
Stoichiometry problems:
- How much of a product can be formed starting from a certain amount of reactants
- How much of one reactant is necessary to react with a given amount of another
- How much reactant is required to produce a desired amount of product
How to solve stoichiometry problems:
- Balancing the chemical equation
- Calculate the molar masses of reactants and products of interest
- Convert all given masses to moles
- Use the balanced equation to determine the stoichiometric ratios
- Calculate the number of moles of desired materials
- Calculate the masses of desired materials
Limiting Reactant
Limiting reactant:
The reactant that is consumed completely in a chemical reaction. The limiting reactant determines the maximum amount of product that can be formed during a reaction
How to determine the limiting reactant:
a A + b B → c C
- Consider one of the starting reagents as the limiting reactant (for example A)
- Calculate the mole number of the other reagent, B, required for a complete reaction of A
Be sure to use the stoichiometric ratios of the balanced equation: = - Compare the amount of B needed for a complete reaction with the actual amount of B:
If amount of B needed > actual amount of B, B is the limiting reactant
If actual amount of B > amount of B needed, A is the limiting reactant
Percent Yield
Theoritical vs. actual yield:
Theoritical yield: the amount of product that will form if all the limiting reactant is consumed
Actual yield: the amount of product actually recovered
Percent yield:
A measure of the efficiency of a chemical reaction. The percent yield is calculated as follows:
% yield = x 100