How are the amounts of substances in reactions calculated? Flashcards
What is relative atomic mass?
atoms have a very small mass so instead of using their actual masses their relative atomic masses are used
(the relative atomic mass is usually in elements shown in periodic table)
What is the definition of relative atomic mass?
the mass of an atom compared to the mass of an atom of carbon 12
What is relative formula mass (Mr)?
it is used for substances made up of molecules
it is calculated by adding up the relative atomic masses of all the atoms in the formula for the substance
How do you work out relative formula mass?
1) work out how many atoms of each element are in the formula
2) add together the relative atomic masses for all the atoms of each element
E.G. the formula for carbon dioxide is CO2. it has one carbon atom (relative atomic mass = 12.0) and two oxygen atoms (relative atomic mass = 16.0):
Mr of CO2 = 12.0 + 16.0 + 16.0 = 44.0
that is the relative formula mass
What is avagadros constant?
one mole contains the same number of prticles as there are in 12g of carbon 12
as atoms and molecules are small, the number of particles in the mole is very large
AVAGADROS CONSTANT = 6.0 x 10^23 atoms
How do you find out the number of particles of a substance?
number of particles = 6.0 x 10^23 x number of moles
E.G.
calculate the number of water molecules in 0.5 moles of water
number of water molecules = 6.0x10^23 x 0.5 = 3.0 x 10^23
How do yo calculate the number of moles?
number of moles = mass / relative atomic mass
E.G.
Calculate the number of moles of carbon atoms in 6.0g of carbon
number of molecules = 6.0 / 12.0 = 0.50 mol
What is limiting and in excess?
a reaction finishes when one of the reactants is all used up
the other reactant has nothing left to react with so some of it is left over
the reactant is all used up is called the limiting reactant
the reactant that is left over is in excess
