topic 3: quantitive chemistry Flashcards
how do you work out the relative formula mass of a compound?
you add together all the relative atomic masses of all atoms in the compound.
how do you calculate the % mass of an element within a compound? (formula)
percentage mass of an element in a compound =
relative atomic mass (Ar) x no. of atoms in element/
relative atomic mass (Mr)
all x100
what is avogadro’s constant?
6.02 x 10^23
what is a mole of a substance?
an amount that contains avogadro’s number of particles (6.02 x 10^23)
why do we use avogadro’s number?
because one mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass, e.g. carbon has an Ar of 12, so one mole of it weighs exactly 12g
how do you work out the number of moles in a given mass? (formula)
number of moles =
mass in g (of the compound or element)/
Mr (of the compound or element)
what dot he big numbers in front of chemical formulas mean?
the number of moles of each substance takes part in a reaction
e.g. Mg + 2HCl –> MgCl + H2
means 1 mole of Mg reacts with 2 moles of HCl to form 1 mole of MgCl and 1 mole of H2
what is a limiting reactant?
a reactant that gets completely used up before the other reactants in a reaction thus will stop the reaction from carrying on
what rule links the limiting reactant and the products formed?
the amount of products formed is directly proportional to the amount of limiting reactant
e.g. if you halve the amount of limiting reactant, the amount of product formed will also halve
how do you work out the volume of a known mass of any gas (at room temp)?
volume of a gas (dm³) =
mass of gas (g) / Mr of gas x 24
why do we times by 24 when finding the volume of a gas?
because at room temp (20°) and pressure (1 atm) one mole of ANY gas occupies 24dm³
