Quantitative Chemistry Flashcards
What does Ar stand for?
Ar stands for the relative atomic mass.
Example:
24
Mg
12
The Ar for magnesium would be 24.
Calculate the relative atomic mass for MgCl2:
The Mr of MgCl2 = 95
Method:
Ar of Mg = 24
Ar of Cl = 35.5
Mg + (2 x Cl) = 24 + (2 x 35.5) = 95
- Look at pg. 41 in CGP book to see clearer
What is the calculation for calculating the percentage mass of an element in a compound?
Percentage mass of an element in a compound = (Ar x number of atoms of that element) ÷ (Mr of the compound) x 100
- Look at pg. 41 in CGP book to see clearer
What does the Avogadro constant show?
The Avogadro constant shows that there are 6.02 x 10^(23) particles in a mole.
What is one mole?
One mole of any substance is just an amount of that substance contains an Avogadro number of 6.02 x 10^(23) particles.
What do particles represent?
Particles could be atoms, molecules, ions or electrons.
What is the formula to find the number of moles in a given mass?
Number of moles = (mass in g of an element or compound) x (Mr of the element or compound)
- Look at pg. 42 in CGP book to see clearer
What do we mean when we say the mass is always conversed?
During a chemical reaction, no atoms are destroyed and no atoms are created. This means the same number and type of atoms are on each side of a reaction equation. Because of this, no mass is gained nor lost.
Why does the mass sometimes change in an experiment of an unsealed reaction vessel during a reaction?
- Reason 1: If mass increases it’s most likely one of the reactants is a gas that’s found in the air.
- Reason 2: If the mass decreases it’s most likely one of the products is a gas and all the reactants are solids, liquids or aqueous.
Write a balanced symbol equation for this reaction: 8.1 g of zinc oxide (ZnO) reacts completely with 0.60 g of carbon to form 2.2 g of carbon dioxide and 6.5 g of zinc.
Ar(C) = 12, Ar(O) = 16, Ar(Zn) = 65
Answer: 2Zn + C —> CO2 +2Zn
Method:
1) Work out Mr for each of the substances in the reaction:
ZnO: 65 + 16 = 81
C: 12
CO2: 12 + (2 x 16) = 44
Zn: 65
2) Divide the mass of each substance by it’s Mr to calculate how many moles of each substance reacted or were produced:
ZnO: 8.1/81 = 0.10 mol
C: 0.6/12 = 0.050 mol
CO2: 2.2/44 = 0.050 mol
Zn: 6.5/65 = 0.10 mol
3) Divide by the smallest number of moles which is 0.050:
ZnO: 0.10/0.050 = 2.0
C: 0.050/0.050 = 1.0
CO2: 0.050/0.050 = 1.0
Zn: 0.10/0.050 = 2.0
4) Then place the numbers in front of their symbol in the equations:
ZnO + C —> CO2 + Zn
To become:
2ZnO + C —> CO2 + 2Zn
What is a limiting reactant?
If one reactant gets completely used up in a reaction before the rest, then the reaction will stop - that is the limiting reactant.
What is the relationship between the limiting reactant and the formed product?
Why is this?
The amount of limiting reactant is directly proportional to the amount of product formed. This is because if you add more reactant there will be more reactant particles to take part in the reaction, which means more product particles.
How can you calculate the mass of a product formed in a reaction by using the limiting reactant and the balanced reaction equation?
1) Write out the balanced equation
2) Work out the relative formula masses of the reactant and 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 - that’s how many moles of product will be made of this many moles of reactant
5) Use the number of moles to calculate mass
Example:
Calculate the mass of aluminium oxide formed when 135g of aluminium is burned in air
* 4Al + 3O2 —> 2Al2O3
* Al: 27 Al2O3: 102
* 135/27 = 5
* 4Al moles produce 2Al2O3 - half the number of moles are produced so 5 moles of Al will produce 2.5 moles of Al2O3
* mass = moles x Mr = 2.5 x 102 = 255g
What is a theoretical yield?
Masses you calculate (e.g. the limiting reactant equation).
