Unit 3 - Quantitative Chemistry - Law of conservation of mass Flashcards
State the law of conservation of mass
The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
Explain what the law of conservation of mass means
This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation
Mass is always ______ in chemical reactions
Mass is always conserved in chemical reactions
What are the reactants
The reactants are the substances we start with before a chemical reaction
What are the products
The product is a substance made as a result of a chemical reaction
Mg + Cl2 —> MgCl2
24g of magnesium reacted with 71g of chlorine
Calculate the mass of magnesium chloride produced
24 + 71 = 95g
MgO + 2HCl –> MgCl2 + H2o
A mass of magnesium oxide reacts with 73g of hydrogen chloride to produce 95g of magnesium chloride and 18g of water
Calculate the mass of magnesium oxide in the reaction
95 + 18 = 113g
113g - 73g = 40g
MgO = 40g
State the total number of atoms of Na2CO3 and the number of each atoms of each element
Total number of atoms = 6
Na = 2
C = 1
O = 3
Balance the following equation
Fe2O3 + 3CO —> 2Fe + ___CO2
Fe2O3 + 3CO —> 2Fe + 3CO2
Define relative formula mass
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
Describe what the sum of the relative formula masses of the reactants in the quantities shown in a balanced equation equals
In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.
Explain why Carbon-12 is given a ‘mass’ of exactly 12 units
Carbon-12 is given a ‘mass’ exactly 12 units because it has six protons and six neutrons
Define relative atomic mass
The relative atomic mass of an element is the average mass of the isotopes of that element weighted to take into account the abundance of each isotope
State the symbol for Relative Atomic Mass
Ar (subscript r)
What atom is the standard atom used that all the other atoms in the periodic table are compared to regarding atomic mass
The standard atom used is Carbon-12 and all the other atoms in the periodic table are compared to this regarding atomic mass
E.g. Hydrogen has a relative atomic mass of 1, as most of its atoms have a mass that is 1/12 of the mass of a Carbon-12 atom.
Oxygen is 16 times bigger than 1/12 or Carbon-12
State the equation for the relative atomic mass
Relative atomic mass (Ar) = total mass of all atoms / total number of atoms (total abundance)
total mass of all atoms = (mass number of isotope 1 x percent abundance of isotope 1) + (mass number of isotope 2 x percent abundance of isotope 2)
total abundance/total number of atoms = 100
Calculate the relative atomic mass of Chlorine
Isotope - Chlorine-35 and Chlorine-37
Abundance 75.8% and 24.2% respectively
Give your answer to 3s.f.
Ar = (35 x 75.8) + (37 x 24.2) / 75.8 + 24.2
Ar = 3548.4 / 100
Ar = 35.5 (to 3 s.f.)
Calculate the relative atomic mass of the element Fluorine
Isotopes = Fluorine-19
Abundance = 100.0%
Give your answer to 3s.f.
Ar = (19 x 100) / 100
Ar = 1900 / 100
Ar = 19.0 (to 3s.f.)
State the symbol for Relative Formula Mass
Mr (subscript r)
Rules for relative formula mass
Relative formula mass has no units
Relative formula mass never involves big numbers
State the relative formula mass of methane
Mr of CH4
12 + (4 x 1) = 16
State the relative formula mass of 2CH4
Mr of 2CH4
12 + (4 x 1) = 16
You dont include the big 2.
Calculate the Mr of CaSO4
Mr of CaSO4
40 + 32 + (4 x 16) = 136
Calculate the Mr of Mg(OH)2
Mr of Mg(OH)2
Mr = 24 + (2 x 16) + (2 x 1) = 58
magnesium + sulfuric acid —> magnesium sulfate + hydrogen
Magnesium and sulfuric acid have a combined Mr of 122
Calculate: the total Mr of magnesium sulfate and hydrogen
Mr of MgSO4 and H2 = 122 (because of the law of conservation of mass)
What is percentage mass
Percentage mass tells us the percentage of an element in a compound or molecule
State the formula percentage mass
Percentage by mass = Mass of element / Mass of compound x 100
What percentage of magnesium oxide is actually magnesium
Percentage by mass = 24/(24 + 16) x 100 = 60%
Calculate the percentage by mass of calcium in calcium chloride
Mr CaCL2 = 111
Ar Ca = 40
Percentage by mass = Mass of element / Mass of compound x 100
Percentage by mass = 40/111 x 100
Percentage by mass = 36%
Calculate the percentage by mass of hydrogen in methane
Mr CH4 = 16
Ar H = 1
Percentage by Mass of H in CH4 = 4 / 16 X 100
= 25%
What does 1 mole of any substance contain
1 mole of any substance contains the same number of atoms
What is 1 mole
1 mole is the amount of substance in grams which has the same number of particles as there are atoms in 1g of Carbon-12
State the unit for moles
Unit for moles = mol
What is the mass of one mole of a substance in grams numerically equal to?
The mass of one mole of a substance in grams is numerically equal to its relative formula mass.