Chemical Calculations (2, 3, 4, 5) Flashcards
math but we suck at it
What is the definition and value of the Avogadro constant?
The amount of particles in one mole of a substance:
6.02 × 10²³
What are relative atomic mass (Aᵣ) and relative molecular mass (Mᵣ)?
The ratio of the average mass of an atom/molecule to ¹⁄₁₂ the mass of one carbon-12 atom
Relative formula mass (also Mᵣ) is the proper term for ionic compounds
What is the definition and formula of molar mass?
The mass of one mole of a substance:
molar mass (g/mol) = mass (g) / number of moles (mol)
What is the definition and formula of percentage composition?
The percentage by mass of an element in a compound:
% composition = (number of atoms of element × Aᵣ of element / Mᵣ of compound) × 100%
Aᵣ and Mᵣ can be replaced with their respective molar masses
What is the definition of an empirical formula?
The simplest formula of a compound
e.g. empirical formula of C₆H₁₂O₆ is CH₂O, all numbers are divided by 6
How do you calculate a compound’s empirical formula?
Given the percentage composition of each element
- Let total mass be 100g
- Find number of moles of each element
- Find mole ratio of elements
- Find ratio of elements (round mole ratios to whole numbers if they are close)
- (not always necessary) multiply ratio to obtain whole numbers
it is recommended to present calculations in a table; refer to pg 33
How do you calculate a compound’s molecular formula?
When given its molecular mass and empirical formula
- Let n be the number multiple of the subscript in its empirical formula e.g. (CH₂O)ₙ
- Calculate molecular mass of empirical formula
- n = molecular mass of compound / molecular mass of empirical formula
- Multiply empirical formula subscripts by n
e.g. molecular formula = (CH₂O)₆ =C₆H₁₂O₆
What is the volume of 1 mole of gas at r.t.p.?
24 dm³
Hence a gas’s density is 24dm³/mol, not 24mol/dm³!
What is the definition of a limiting reactant?
The reactant that is used up first in a reaction
How do you calculate the limiting reactant?
- Find mole ratio of reactants
- Determine amount of moles of first reactant needed to completely react the second reactant
- Based on amount of first reactant given, determine if it is the limiting reactant or in excess (other reactant is limiting reactant)
if more is given than is needed, it is in excess. else, it is the limiting reactant
When amount of only one reactant is given, can assume there is no excess
What are two assumptions made in chemical calculations using stochiometric ratios?
Calculating mass of reactants/products, moles of reactants/products etc.
- Reactants are 100% pure substances
- All reactants are converted to products
What is the definition and formula of percentage yield?
The percentage by mass OR moles of a product that is obtained compared to how much would ideally be obtained:
% yield = (actual yield (g OR mol) / theoretical yield (g OR mol) ) × 100%
Actual yield and theoretical yield must be in the same units
What is the definition and formula of percentage purity?
The percentage by mass of a substance that is pure/reacted:
% yield = (amount of substance reacted (g) / amount of substance used (g)) × 100%
What is the definition and formula of mass concentration?
The amount of grams of solute in one dm³ of solution:
mass concentration (g/dm³) = mass (g) / volume (dm³)
OR
mass concentration (g/dm³) = molar concentration (mol/dm³) × molar mass (g/mol)
What is the definition and formula of molar concentration?
Also known as the molarity of a solution
The amount of moles of solute in one dm³ of solution:
molar concentration (mol/dm³) = number of moles (mol) / volume (dm³)
OR
molar concentration (mol/dm³) = mass concentration (g/dm³) / molar mass (g/mol)
When asked for molarity, use M as units instead of mol/dm³
