Calculations (1) Flashcards
What is an isotope
Atoms of the same element with the same number of protons but a different number of neutrons
What would happen if isotopes had different numbers of protons
They would be different elements all together
Isotopes have the same ___________ number but different ________ numbers
Atomic numbers
Mass number
What is relative atomic mass
A way of saying how heavy different atoms are
Relative atomic mass is the average _______ of all the isotopes of an element. It has to allow for the relative ______ of each element and its relative _____________
Mass
Mass
Abundance
How do you figure out the relative atomic mass of multiple isotopes
Multiply the mass of each isotope by its relative abundance
Add those together
Divide by the sum of the relative abundances (which is always 100 (25+75=100))
You can find the relative atomic mass of any element using the periodic table. Relative atomic masses don’t usually come out as whole numbers so they’re often rounded to the nearest _____ in periodic tables
0.5
Relative formula mass is also known as ______
Mr
What is relative formula mass
The relative atomic masses in a compound added up to make one big mass
What does empirical formula give you
The smallest whole number ratio of atoms in a compound.
How to do empirical formula
List all the elements in the compound
Underneath them write their masses or percentages(from question)
Divide each question mass or percentage by Ar
Turn the numbers you get into a ratio
Get ratio to simplest form
Fe
- 8
- 8 / 56 = 0.8
O
- 2
- 2 / 16 = 0.12
0.8 : 0.12 = 2 : 3
What does molecular formula give you
The actual number of atoms of each element in a single molecule
How do you find the molecular formula. E.g A molecule has an empirical formula of … and a relative formula mass of…. Work out the molecular formula.
Find the mass of the empirical formula
Find how many empirical units there are in the molecule
(If the relative molecular mass (from question) is 166 and the empirical formula mass is 83 then there are 2 empirical units)
How to calculate masses in reactions
Write out the balanced equation
For the 2 bits you want, work out the Mr and multiply them by the balancing numbers in the equation
Apply the rule: Divide to get one, then multiply to get all
So divide to get 1 gram of something and then multiply it to get the amount you want
(apply it first to the substance given)
_______________________________________
What mass of magnesium oxide is produced when 60g of magnsium is burnt in air
2mg + O2 —> 2MgO
So if you have 2Mg then the Mr of this is 48
You need to find how much of this is produced when this 60g of magnesium is burnt in air. So divide it by 48 to get 1 and then multiply by 60 to find 60g of it. Do the same to the 2MgO and you’ll find how much 2MgO will be produced.
______________________________________
If the question had asked what mass of 2Mg would be produced from 500g of magnesium oxide, then do it the opposite way around. Divide the 2Mgs by its Mr and then multiply it by 500
Percentage yield formula
Percentage yield = actual yield / theoretical yield x 100
