Formulae, Equations & Avogadro (Edexcel A Level Chemistry): Flashcards
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Define empirical formula.
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Define empirical formula.
An empirical formula is the simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound.
For example, the empirical formula of ethanoic acid, C2H4O2, is CH2O.
True or False?
The empirical formula and molecular formula of a compound are always the same.
False.
The molecular formula shows the actual number of each atom in a molecule. The empirical formula is the simplest whole number ratio, which may be different — for example, ethanoic acid has molecular formula C2H4O2 but empirical formula CH2O.
What are the steps to determine the empirical formula of a compound from the mass of each element present?
Divide the mass of each element by its relative atomic mass to find the moles of each element. Then divide all values by the smallest to obtain the simplest whole number ratio of atoms.
To find the molecular formula from the empirical formula, divide the .......... of the compound by the .......... of the empirical formula, then multiply each subscript by this value.
To find the molecular formula from the empirical formula, divide the Mr of the compound by the Mr of the empirical formula, then multiply each subscript by this value.
Define molecular formula.
A molecular formula is the formula that shows the number and type of each atom in a molecule.
For example, the molecular formula of ethanoic acid is C2H4O2.
What is water of crystallisation and how is it represented in a chemical formula?
Water of crystallisation is water that forms part of a crystal's structure in a hydrated compound. It is separated from the main formula by a dot, for example CuSO4•5H2O.
What is a hydrated compound?
A hydrated compound is a compound that contains water of crystallisation as part of its crystal structure, shown by a dot in its formula — for example, CuSO4•5H2O.
% mass of an element = (.......... × number of atoms of the element ÷ .......... of the compound) × 100
% mass of an element = (Ar × number of atoms of the element ÷ Mr of the compound) × 100
True or False?
The percentage by mass of calcium in CaCO3 is 40%.
True.
Ar of Ca = 40.1, Mr of CaCO3 = 100.1. % mass = (40.1 × 1) ÷ 100.1 × 100 ≈ 40%.
What is a symbol equation?
A symbol equation is a shorthand representation of a chemical reaction using chemical symbols to show the number and type of each atom in the reactants and products.
True or False?
When balancing an equation, you can change the formula of a reactant or product.
False.
When balancing equations, you must only add coefficients in front of formulae. The formulae themselves must never be changed.
What are the steps for balancing a chemical equation?
Write the formulae of reactants and products. Count the atoms of each element on both sides. Adjust coefficients in front of formulae until all atoms are balanced. Finally, add appropriate state symbols.
In chemical equations, (s) = solid, (l) = liquid, (g) = gas and (aq) = ..........
In chemical equations, (s) = solid, (l) = liquid, (g) = gas and (aq) = aqueous (dissolved in water).
What are spectator ions?
Spectator ions are ions in solution that do not take part in a chemical reaction. They appear unchanged on both sides of the equation.
What does an ionic equation show that a full symbol equation does not?
An ionic equation shows only the ions and particles that take part in the reaction. It omits the spectator ions, which remain unchanged in solution.
To write an ionic equation, break all aqueous ionic compounds into their ........... Then cancel any .......... that appear identically on both sides of the equation.
To write an ionic equation, break all aqueous ionic compounds into their ions. Then cancel any spectator ions that appear identically on both sides of the equation.
True or False?
During a chemical reaction, atoms cannot be created or destroyed.
True.
This is the principle behind balancing equations: the number of atoms of each element must be equal on both sides of the equation.
What is a displacement reaction?
A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound.
True or False?
In a neutralisation reaction, an acid and a base react to form a salt and water.
True.
For example: HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l). Some neutralisations also produce CO2 (g) if a carbonate is involved.
How can a balanced chemical equation be used to identify the type of reaction taking place?
The reactants and products in a balanced equation indicate the reaction type: a displacement reaction shows one element replacing another; a neutralisation produces a salt and water; a precipitation produces an insoluble solid from two aqueous reactants.
A reaction between two aqueous solutions that produces an .......... solid product can be identified as a .......... reaction.
A reaction between two aqueous solutions that produces an insoluble solid product can be identified as a precipitation reaction.
Why does an ionic equation represent a precipitation reaction more clearly than a full symbol equation?
An ionic equation shows only the ions that form the precipitate, removing the spectator ions. This makes it clear which species are directly responsible for the insoluble solid product forming.
What is a precipitation reaction?
A precipitation reaction is a reaction in which two aqueous solutions react to form an insoluble solid (a precipitate) as one of the products.
In a neutralisation reaction between a strong acid and a strong base, a hydrogen ion reacts with a .......... ion to form ..........
In a neutralisation reaction between a strong acid and a strong base, a hydrogen ion reacts with a hydroxide ion to form water.
True or False?
Spectator ions are omitted from ionic equations.
True.
Ionic equations show only the reacting particles. Any ions that are unchanged in both the reactants and products are the spectator ions and are omitted.
What is the Avogadro constant?
The Avogadro constant (NA) is the number of particles in one mole of a substance. Its value is 6.02 x 1023 mol-1.
True or False?
One mole of any element has a mass equal to its relative atomic mass in grams.
True.
For example, one mole of carbon has a mass of 12.0 g and one mole of oxygen atoms has a mass of 16.0 g.
How many moles are in 100 g of water, H2O?
Molar mass of H2O = (2 x 1.0) + 16.0 = 18.0 g mol-1. Moles = mass / molar mass = 100 / 18.0 = 5.56 mol (3 s.f.).
What is molar mass?
Molar mass is the mass of one mole of a substance, measured in g mol-1. For an element, it is numerically equal to the relative atomic mass in grams.
The amount in moles of a substance = .......... / .........., where the denominator has units of g mol-1.
The amount in moles of a substance = mass / molar mass, where the denominator has units of g mol-1.
How do you calculate the number of molecules in a given mass of a substance?
Divide the mass by the molar mass to find moles. Then multiply moles by the Avogadro constant (6.02 x 1023 mol-1) to find the number of molecules.
Define the mole.
A mole is the amount of substance that contains 6.02 x 1023 particles, the same number of atoms as in exactly 12.00 g of carbon-12. Its symbol is mol.
True or False?
The molar mass of ethanol, C2H5OH, is 46 g mol-1.
True.
Molar mass = (2 x 12.0) + (5 x 1.0) + 16.0 + 1.0 = 46.0 g mol-1.
What is amount of substance?
Amount of substance is the quantity that describes the number of particles in a sample, measured in moles (mol). It can be calculated using mass, concentration, number of particles or the ideal gas equation.
True or False?
Moles can be calculated by multiplying concentration by volume.
True.
The equation n = concentration x volume is used when the concentration (in mol dm-3) and volume (in dm3) of a solution are known.
Moles, n = concentration x .......... and also n = .......... / molar mass
Moles, n = concentration x volume and also n = mass / molar mass
What are the steps to calculate the reacting mass of a product using a balanced equation?
Find the moles of the given substance using n = mass / molar mass. Use the molar ratio from the balanced equation to find moles of the target substance. Multiply by its molar mass to obtain the mass.
To convert moles to mass, multiply moles by the substance's ........... To convert mass to moles, .......... the mass by the molar mass.
To convert moles to mass, multiply moles by the substance's molar mass. To convert mass to moles, divide the mass by the molar mass.
How can you determine a balanced equation from the masses of reactants and products?
Convert all masses to moles by dividing by the molar mass. Simplify the mole values to find the simplest whole number ratio of each substance, which gives the balanced equation coefficients.
True or False?
Reacting masses are always in proportion to the coefficients in the balanced equation.
True.
The molar ratio from a balanced equation directly gives the ratio of amounts of each substance. This ratio holds regardless of whether you work in grams, tonnes or any other mass unit.
Which equation is used to calculate the moles of a gas from its pressure, volume and temperature?
The ideal gas equation PV = nRT, where P is pressure (Pa), V is volume (m3), T is temperature (K) and R = 8.31 J mol-1 K-1. Rearranging gives n = PV / RT.
True or False?
When using the ideal gas equation, pressure should be entered in kPa.
False.
Pressure must be in pascals (Pa) in the ideal gas equation. Convert kPa to Pa by multiplying by 1000.
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