Chemistry Test 2
The Law of Conservation of Mass
total mass of reactants = total mass of products
If the total mass of reactants in a chemical reaction is 20g, what can we say about the total mass of products?
Equal to 20g
what is the relative formula mass equal to (Mr)
Relative Atomic Mass (Ar)
The relative atomic mass is the
average mass of the atom (sum of the relative atomic masses)
The relative formula mass (Mr) is the sum of the
relative atomic masses (average mass of the atom) of every atom in a formula.
Remember how we work out relative formula mass
Mr = Sum of (Ar of element x number of atoms in element)
what is the Ar of an element
Relative atomic mass: The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes
What is the Mr of an element
Relative formula mass: The sum of the relative atomic masses of all the atoms in a chemical formula
When calculating the Mr of a compound, you must first identify what
the Ar of each element in the compound.
What are the steps when calculating the relative formula mass of a compound?
1 Identify the Ar of each element
2 Multiply the Ar by the number of atoms
3 Add up the totals for each element
Substances can seem to lose or gain mass when they react. This is usually because
a reactant or product is a gas.
what equation links mass, molar mass and moles
moles x molar mass = mass
Determine the mass of iron oxide produced when 28g of iron is burnt in air
STEP 1: 2Fe + 1.5O2 → Fe2O3
STEP 2: moles of iron = mass/Mr 28g / 56 = 0.5moles
STEP 3: Molar ratio 2Fe: Fe2O3 0.5: 0.25
STEP 4: Mass of iron oxide = moles x Mr
Mr of Fe2O3 = (2x56) + (3x16)
= 160
0.25 x 160 = 40g
what is a mole
it is used to measure the particle in a substance
how to calculate the num of paricles/atoms
moles x avagadro's constant
When 5.0 g of calcium carbonate is decomposed by heating, it produces 2.2 g of carbon dioxide. What mass of calcium carbonate is needed to produce 8.8 g of carbon dioxide?
5 g of calcium carbonate makes 2.2 g of carbon dioxide.
5 ÷ 2.2 g of calcium carbonate will make 1 g of carbon dioxide.
Therefore, to make 8.8 g of carbon dioxide, you will need 8.8 × (5 ÷ 2.2 g) = 20 g of carbon.
what are you trying to find for Reacting mass calculations
Chemical / symbol equations can be used to calculate: The moles of reactants and products. The mass of reactants and products.
When 12 g of carbon is burned in air, 44 g of carbon dioxide is produced. What mass of carbon is needed to produce 11 g of carbon dioxide?
12 g of carbon makes 44 g of carbon dioxide.
12 ÷ 44 g of carbon will make 1 g of carbon dioxide.
Therefore, to make 11 g of carbon dioxide, you will need 11 × (12 ÷ 44 g) = 3 g of carbon.)
what is the reactant
is all that is all that is used up (limiting reactant) e.g the excess
what is the percentage yield
the percentage yield of a reaction is a comparison between theoretic yield and the amount you actually get
what is the theoretic yield
the theoretical yield of a reaction is the amount you would get if all reactants formed the desired products and none of the products were lost e.g 10g iron + 10g sulfur = 20g iron sulfide
how do you calculate the percentage yield
percentage yield = mass of products actually made(g) / max theoretical yield mass(g)
how do you calculate the atom economy
Atom Economy = Mr of useful product/Mr of all products x 100
how do you calculate the concentration
concentration = mass dissolved(mol)/vol of solution(dm³)
what are the units for concentration
mol/dm³
what equation that links mass, Mr and moles
mass = moles x Mr
what equation that links moles, concentration and volume
concentration = moles/volume (vol on top of triangle)(conc on left)
how do you convert cm³ to dm³
divide by 1000
what is titration used for
determining the concentration of an unknown substance in a sample
what does avogadro's law state
equal volumes of gases, at the same temp and pressure, contain the same number of molecules
What features of a reaction make it more likely to be incomplete?
it is slow
it is reversable
The theoretical yield can be calculated from a balanced equation if we know the mass of the
reactants.
Which term is used to describe the amount of the reactants that end up as useful products?
atom economy
what double equation do you use for titration (Ratio)
moles = conc x volume
moles = conc x volume
how do you calculate the % of a element in a compund
Ar x num of atoms in the element/Mr x 100 (test result one)