topic 9- calculations Flashcards
The relative formula mass is equal to…
The sum of the relative atomic masses of all the atoms in the compound
The relative atomic mass is the…
average mass of the atom
How do we calculate relative formula mass?
We find the relative atomic masses (Ar) of all the elements that make the compound. Then we multiply the Ar values by the number of atoms of that element in the compound. Then we add these numbers together.
Calculate the Mr of sulphuric acid (H₂SO₄):
Ar of hydrogen= 1
Ar of sulphur= 32
Ar of oxygen= 16
Mr of H₂SO₄= (2x1)+(1x32)+(4x16)=98
What is the empirical formula of an ionic compound?
The simplest ratio of ions possible
Molecular formula
Shows the actual amount of atoms which make up a molecule
Calculating empirical formula:
- mass (given in question)
- divide by Ar
- divide by smallest
- formula
The law of conservation of mass:
Total mass of reactants= total mass of products
In a chemical reaction atoms cannot be…
Destroyed or created
Equation for number of moles:
n(mol)= m (g) moles= mass
——— ———
Mr (g/mol) relative formula mass
Examples of reactions in which mass changes:
- corrosion of metal
- zinc + hydrochloric acid
- burning of wood
Triangle for mass, concentration and volume:
Mass
Volume x Concentration
Equations for concentration:
- Concentration (g/dm³) = mass (g) / volume (dm³)
- Concentration (mol/dm³)= moles (mol)/ volume (dm³)
What is Avogadro’s number?
6.02x 10²³
what is a mole?
1 mole of any substance is 6.02x 10²³ atoms of that substance
how do we know what 1 mole is of a substance?
- 1 mole= the relative formula mass (Mr) of a substance
- for elements this is the same as the relative atomic mass
balance the equation: H₂ + O₂ —> H₂O
2H₂ + O₂ —> 2H₂O
limiting reactant
the reactant that is used up is called the limiting reactant because the reaction stops when it is used up
How do we make sure that no acid remains after the reaction?
By using an excess (‘more than necessary’) amount of the other reactant
How do we reduce the uncertainty of results?
We can take multiple readings and calculate an average from them
Method of empirical formula:
Find the mass of oxygen Divide the masses of all elements Divide by the smallest number Make the numbers whole numbers State the empirical formula
Equation for the percentage atoms economy:
Mr (desired product)
———
Sum of Mr (for all reactants)
