topic three: quantitative chemistry Flashcards
what is the relative formula mass?
the sum of the relative atomic masses of all the elements in the formula
what is a mole?
the measurement of the amount of a substance
what is avogadro’s constant
the number of atoms, molecules of ions in one mole of a give substance
what is the value of avogadro’s constant?
6.02 x 10^23
what is the formula for working out the number of moles?
moles = mass(g) / mr
define: conservation of mass
the total mass of reactants must be equal to the total mass of products
what would explain an increase of mass in a reaction?
one of the reactants must be a gas that was not previously measured
what would explain a decrease of mass in a reaction?
one of the products was a gas that escaped
how would you find the amount of moles of a substance in a reaction?
you can use molar ratios to find the amount of moles of a substance in a reaction
in the reaction:
N2 + 3H2→ 2NH3
what is the molar ratio?
1 : 3 : 2
define: an excess
when the amount of reactant is greater than the amount that can react
define: limiting reactant
the reactant in a reaction that determines the amount of products formed
give three reasons why we don’t get 100% percentage yield
- some reactions do not go to completion (e.g. reverse reactions)
- some of the product may have been lost (e.g. left on apparatus)
- some reactants react differently than expected and so do not contribute to the products
define: percentage yield
the amount of product formed in a reaction compared to the maximum theoretical mass
give the formula for percentage yield
percentage yield = 100 x
mass of product actually made
——————————————–
theoretical mass of products
