Topic 3 - Quantitive Chemistry Flashcards
How do you calculate the Relative Formula Mass of a compound?
You simply add the relative atomic masses of the substances within the compound together
Example Question: Find the Relative Formula Mass of MgCl2
Relative Atomic mass of Mg: 24
Relative atomic mass of Cl: 35.5
Mg + (2 X Cl) = 24 + (2 X 35.5) = 95
So the Relative Formula Mass of MgCl2 is 95
How do you calculate the Percentage Mass of an Element in a compound
(Formula)
Percentage mass of an element in a compound =
Relative Atomic Mass of the element
__________________________________________ X 100
Relative Formula Mass of the compound
What is the Avogadro constant
It is basically a number like 1 million, but is equal to:
6.02 x10(23)
How many particles are in one mole of any substance and why is it that number
The Avogadro constant
6.02 x10(23)
It is this number as one mole of an atom or molecule of any substance will have the same mass in grams equal to the relative formula mass for that substance
E.g carbon’s relative atomic mass is 12, so one mole of carbon weighs 12 grams
What is the formula for the number of moles in a given mass
Number of moles =
mass in grams (of an element or compound)
________________________________
Relative formula mass (of the element or compound)
Example question: How many moles are there in 66g of carbon dioxide
Mr of CO2 = 12 + (16 x 2) = 44
Moles = 66/44 = 1.5 moles
Example question: What mass of carbon is there in 4 moles of carbon dioxide
4 x 12 = 48
What is the conservation of mass?
During a chemical reaction, no atoms are destroyed and no atoms are created
What does it mean if there is a change in mass during a chemical reaction
This means that a gas is being released or involved
- If the mass increases, a gas which was present at the start of the reaction, and becomes a solid, liquid or aqueous solution. It becomes part of the product
- If the mass decreases, one of the products is a gas, as a reactant which is a solid, liquid or aqueous solution changes. The gas will only escape if the vessel isn’t enclosed
How many moles of each substance are used in this equation:
Mg + 2HCl –> MgCl2 + H2
1 mole of Mg
2 moles of HCl
1 mole of MgCl
1 mole of H
How would you work out a balanced symbol equation for a reaction if you only know the masses of the reactants and products? (4 Steps)
1) Divide the mass of each substance by its relative formula mass to find the number of moles
2) Divide the number of moles of each substance by the smallest number of moles in the reaction
3) If any of the numbers aren’t whole numbers, multiply all numbers by the same amount so that they all become whole numbers
4) Write the balanced symbol equation for the reaction by putting the numbers in front of the chemical formulas
Example question: 8.1g of Zinc oxide (ZnO) react with 0.6 g of carbon to form 2.2g of carbon dioxide and 6.5g of Zinc. Write a balanced symbol equation for this reaction
- Work out the Mr
ZnO: 65 + 16 = 81. C = 12. CO2 = 44. Zn = 65 - Divide the mass of each substance by its Mr (to calculate the moles)
ZnO = 0.1. C = 0.05. CO2 = 0.05. Zn = 0.1 - Divide by smallest moles (which is 0.05)
ZnO = 2. C = 1. CO2 = 1. Zn = 2 - Write the balanced symbol equation using these whole numbers
2ZnO + C = CO2 + 2Zn
What makes a reactant a ‘limiting factor’
This is when a substance is completely used up during a reaction before the rest of the reactions, and the reaction therefore stops
It is directly proportional to the product e.g double the limiting reactant = double the product amount
How do you calculate the mass of a product formed in a reaction by using the mass of the limiting reactant and the balanced reaction equation? (5 Steps)
1) Write out the balanced equation
2) Work out the relative formula masses of the reactant and the product you want
3) Find out how many moles there are of the substance you know the mass of
4) Use the balanced equation to work out how many moles there’ll be of the other substance
5) Use the number of moles to calculate the mass
