Quantitative chemistry Flashcards
Mr
Relative formula mass
Mass of all atoms in formula
Ar
Relative atomic mass
Mass of individual atoms
Relative formula mass
Mr
The relative atomic mass of a compound
Relative atomic mass
Ar
An average mass of an element
RAM of an Element equation
RAM=
sum of (atomic mass x % of each element
————————————————————
100
RAM of an element example
Copper 63 70% copper 65 30%
63 x 70 = 4410
65 x 30 = 1950
= 6360
——- = 63.6
100How to find out the percentage mass of an element in a compound
Ar x number of atoms of that element
—————————————x100
Mr of the compound
% mass of an element in a compound example
Find % mass of Na in Na2CO3
Na= 23(46) C=12 O= 16(48) Mr= 106
46
—- x 100 = 43%
106
Mole
An amount of substance
Carbon has an Ar of 12
One mole of carbon weighs 12g
Equation linking mass moles and Ar/Mr
Number of moles=
Mass
———
Mr/Ar
Change in mass
Happens when reaction vessel is unsealed
Mass increases- one of the reactants is a gas that’s found in air and all the products are s l or a
Decreases- one of the products is a gas and all the reactants s l or a
Product gas can escape
Big numbers before chemical formula
Number of moles
When do reactions stop
When one reactant is used up
Any other reactant is excess
They’re usually added in excess to make sure that the other is used up
Limiting reactant
The reactant that’s used up
It limits the amount of product that’s formed
Volume of gas equation
Volume =
Moles x 24(000cm3)dm3
Concentration equation
Mass
———
Volume
Moles
———
Volume
Atom economy
Tells you how much of the mass of the reactants is wasted when manufacturing a chemical and how much ends up as the desired product
Atom economy equation
Relative formula mass of desired product
—————————————-x100
Relative formula mass of all reactants
Yield
Amount of product you get
Percentage yield equation
Mass of product actually made
—————————————x100
Max theoretical mass of product
What is the conservation of mass?
The total mass of reactants is equal to the total amount of product
What must we do to chemical equations to show the conservation of mass?
Balance it
Why is % yield never 100%?
Some product could be lost, if it is reversible it may not go through to completion, some reactants may react differently than expected E.g side reaction. And you lose some product when you separate it from the reaction mixture
Why do scientists try and choose pathways with high atom economy?
Economic reasons, sustainable development- more made, less wasted
