Topic 7 - Quantitative Chemistry Flashcards
What is relative formula mass?
The relative formula mass of a compound is the sum of the relative atomic masses in the numbers shown in the formula.
What is the symbol for relative formula mass?
Mr
Has no units.
How to workout relative formula mass?
Multiply the amount of each atom by its relative atomic mass.
Add these for all the atoms together.
Multiply brackets first.
Example of relative formula mass.
Al2O3
Relative atomic masses:
Al = 27
O = 16
(27x2) + (3x16) = 102
Percentage mass by composition.
Defintion
The percentage by mass of an element in a compound tells us what mass of a particular compound is due to a particular element.
Percentage mass by composition.
Equation
Mass of the element in compound / total mass of compound (Mr) X 100
Percentage mass by composition.
Example
H2O2
Calculate percentage by mass of oxygen.
Relative atomic mass
H = 1
O = 16
(1x2) + (16 x 2) = 34
32/34 x 100 = 94.1%
Molecular formula
Represents the actual number of atoms of each element in one molecule.
Empirical formula
The simplest whole number ratio of different atoms of each element in a compound.
Conservation of mass
Mass cannot be created or destroyed during the course of a chemical reaction.
Closed system
When the mass of the reaction does not change.
Open system
When the mass of the reaction does change.
Avogadro’s number:
1 mole of substance
1 mole of substance = 6.02 x 10^23 particles (atoms or molecules or ions) of that substance.
The mass of 1 mole of a substance is the relative atomic mass (Ar) or relative formula mass in grams (Mr).
Equation to calculate the number of moles of a substance?
Number of moles of a substance = Mass of substance (g) / Ar or Mr
Number of particles
Number of particles = Mass X Avogadro’s number
