C3- Quantitative Chemistry- Paper 1 Flashcards
What is the relative formula mass of a compound?
The sum of the relative atomic masses of all the atoms in the compound
It is denoted as Mr
How do you find the relative formula mass of MgCl₂?
Mg + (2 x Cl) = 24 + (2 x 36) = 96
M of MgCl₂ = 95
What is the formula to calculate the percentage mass of an element in a compound?
Percentage mass of an element = (Ar x number of atoms of that element / Mr of the compound) × 100
Ar is the relative atomic mass.
Calculate the percentage mass of sodium in sodium carbonate (Na₂CO₃) given Ar of sodium is 23, carbon is 12, and oxygen is 16.
Percentage mass of sodium = (23 x 2 / 106) × 100 = 43%
M of Na₂CO₃ = (2 × 23) + 12 + (3 × 16) = 106
If a mixture contains 20% iron ions by mass, how much iron is in 50 g of the mixture?
10 g of iron
Calculation: 50 g × 20% = 10 g
What is the percentage mass of iron in iron chloride (FeCl₂) if Ar of Fe = 56 and Ar of Cl = 35.5?
Percentage mass of iron = (56 / (56 + (2 × 35.5))) × 100 = 44.09%
M of FeCl₂ = 56 + (2 × 35.5) = 127
How much iron chloride is needed to provide 10 g of iron?
23 g of iron chloride
Calculation: 10 g of iron / 44.09% = 22.7 g, rounded to 23 g
What is the formula mass of sodium carbonate (Na₂CO₃)?
106
M of Na₂CO₃ = (2 × 23) + 12 + (3 × 16)
What is the first step to calculate the relative formula mass of a compound?
Look up the relative atomic masses of all the elements in the compound.
What is the definition of a mole?
A mole is simply the name given to an amount of a substance equivalent to 6.02 x 10^23 particles
The particles can be atoms, molecules, ions, or electrons.
What is Avogadro’s number?
6.02 x 10^23
This number represents the number of particles in one mole of a substance.
How is the mass of one mole of a substance related to its relative atomic or formula mass?
The mass of one mole of atoms or molecules of any substance is equal to the relative atomic mass (A_r) or relative formula mass (M_r) in grams.
This means that the mass in grams of one mole corresponds directly to the atomic or formula mass.
What is the mass of one mole of carbon (C)?
12 g
Carbon has a relative atomic mass (A_r) of 12.
What is the mass of one mole of nitrogen gas (N2)?
28 g
Nitrogen gas has a relative formula mass (M_r) of 28 (2 x 14).
What is the mass of one mole of carbon dioxide (CO2)?
44 g
Carbon dioxide has a relative formula mass (M_r) of 44 (12 + [2 x 16]).
How do you find the number of moles in a given mass?
Number of moles = mass in g (of an element or compound) ÷ M_r (of the element or compound)
This formula calculates the amount of substance in moles based on its mass.
Calculate the number of moles in 66 g of carbon dioxide (CO2).
1.5 mol
M_r of CO2 is 44; thus, 66 g ÷ 44 g/mol = 1.5 mol.
What is the formula triangle for calculating moles?
mass = no. of moles x Mr
Covering the variable you want to find allows rearranging the formula easily.
What mass of carbon is there in 4 moles of carbon dioxide?
48 g
There are 4 moles of carbon in 4 moles of CO2, so 4 moles x 12 g/mol = 48 g.
True or False: The mass of 12 g of carbon, 28 g of N2, and 44 g of CO2 all contain the same number of particles.
True
Each of these masses represents one mole of the respective substances.
Calculate the number of moles in 90 g of water (H2O).
5 mol
M_r of H2O = 2(1) + 16 = 18; thus, 90 g ÷ 18 g/mol = 5 mol.
Calculate the mass of 0.20 mol of potassium bromide (KBr).
12 g
M_r of KBr = 39 + 80 = 119; thus, 0.20 mol x 119 g/mol = 23.8 g.
What is the principle of conservation of mass?
During a chemical reaction, mass is always conserved, meaning no atoms are destroyed or created.
In a balanced chemical equation, what is true about the number and types of atoms?
The same number and types of atoms are present on each side of the reaction equation.
What happens to mass during a chemical reaction?
No mass is lost or gained; we say that mass is conserved.
Provide an example of a balanced chemical equation that illustrates conservation of mass.
2Li + F2 → 2LiF
How can you verify that mass is conserved in a chemical reaction?
By adding up the relative formula masses of the substances on each side of the balanced equation.
What are the total relative formula masses on both sides of the reaction 2Li + F2 → 2LiF?
52 on both sides.
What typically explains an increase in mass observed in an unsealed reaction vessel?
One of the reactants is a gas from the air that becomes contained within the vessel.
What example illustrates an increase in mass due to a gas involvement?
When a metal reacts with oxygen in an unsealed container.
What typically explains a decrease in mass observed in an unsealed reaction vessel?
One of the products is a gas that escapes from the reaction vessel.
What example illustrates a decrease in mass due to a gas escaping?
When a metal carbonate decomposes to form a metal oxide and carbon dioxide gas.
Fill in the blank: If the mass increases in a reaction, it is probably because one of the reactants is a _______.
gas
True or False: In an unsealed reaction vessel, a gas produced as a product can lead to an increase in the total mass measured.
False
Fill in the blank: A gas will expand to fill any container it’s in; if the reaction vessel isn’t sealed, the gas _______.
escapes
