Formulae, Equations & Calculations (AQA A Level Chemistry): Flashcards
Exam code: 7405
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Define relative atomic mass (Ar).
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Define relative atomic mass (Ar).
A Relative atomic mass is the ratio of the average mass of the atoms of an element to one-twelfth of the mass of a carbon-12 atom.\n\nAr has no units as it is a dimensionless ratio.
True or False?
The unified atomic mass unit is defined as one-twelfth of the mass of a carbon-12 isotope.
True.
This standard is used because the actual mass of a single atom is far too small to measure directly.
The Ar is a .......... average of the masses of the .......... of a particular element, weighted by their .......... abundances.
The Ar is a weighted average of the masses of the isotopes of a particular element, weighted by their percentage abundances.
Define relative molecular mass (Mr).
A Relative molecular mass is the ratio of the weighted average mass of a molecule of a molecular compound to the unified atomic mass unit.\n\nMr has no units and is found by summing the Ar values of all atoms in the molecular formula.
True or False?
Ar and Mr both have units of grams per mole (g mol-1).
False.
Both Ar and Mr are dimensionless ratios — they have no units because the units cancel in the ratio.
For ionic compounds, the .......... .......... mass is used. It is calculated in the same way as Mr but uses the .......... formula unit rather than a discrete molecule.
For ionic compounds, the relative formula mass is used. It is calculated in the same way as Mr but uses the simplest formula unit rather than a discrete molecule.
How is the Ar of an element calculated from isotope data?
Multiply each isotope's mass number by its percentage abundance, sum the results, then divide by 100.
This gives the weighted average mass relative to one-twelfth of the mass of a carbon-12 atom.
Define empirical formula.
An Empirical formula is the simplest whole number ratio of atoms of each element present in a compound.\n\nFor example, the empirical formula of ethanoic acid (C2H4O2) is CH2O. Note that the same empirical formula can belong to many different compounds with different molecular formulae.
True or False?
Ionic compounds do not have a molecular formula — they are represented only by an empirical formula.
True.
Ionic compounds exist as giant ionic lattices, not discrete molecules. They are represented by their empirical formula (the simplest ratio of ions). The term "molecular formula" does not apply to ionic compounds.
Divide the mass of each element by its .......... to get moles. Then divide all mole values by the .......... value to get the simplest whole number ...........
Divide the mass of each element by its Ar to get moles. Then divide all mole values by the smallest value to get the simplest whole number ratio.
Define molecular formula.
A Molecular formula is the formula showing the exact number and type of each atom present in one molecule of a compound.\n\nIt is a whole-number multiple of the empirical formula.
How is the molecular formula calculated from the empirical formula?
Divide the relative molecular mass (Mr) of the compound by the relative mass of the empirical formula.
Multiply each subscript in the empirical formula by this whole-number ratio to give the molecular formula.
True or False?
When percentage composition data is provided for only some elements, you should assume the remaining percentage is zero.
False.
If percentages do not sum to 100 %, the remainder belongs to an unlisted element. Find that element by subtraction before starting the empirical formula calculation.
A hydrocarbon contains 90.0 % carbon and 10.0 % hydrogen by mass. Complete the ratio calculation.
Moles of C = 90.0 / 12.0 = .......... .
Moles of H = 10.0 / 1.0 = .......... .
Ratio C : H = .......... : .......... .
The empirical formula is .......... .
Moles of C = 90.0 / 12.0 = 7.5.
Moles of H = 10.0 / 1.0 = 10.0.
Ratio C : H = 3 : 4.
The empirical formula is C3H4.
What is a spectator ion?
A Spectator ion is an ion present in a reaction mixture that does not take part in the chemical reaction.\n\nSpectator ions are removed when writing an ionic equation.
An ionic compound is electrically .......... . The total positive charge from the .......... ions must equal the total .......... charge from the anions.
An ionic compound is electrically neutral. The total positive charge from the metal ions must equal the total negative charge from the anions.
True or False?
When balancing an equation, you may change the subscripts within a chemical formula to make the atom counts balance.
False.
You must never alter the subscripts within a formula. Only the large number coefficients placed in front of each formula may be changed.
What does the state symbol (aq) indicate in a chemical equation?
(aq) indicates that the substance is dissolved in water (aqueous solution).
The four state symbols are (s) solid, (l) liquid, (g) gas and (aq) aqueous.
Zn (s) + .......... (aq) → .......... (aq) + Cu (s)
Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
SO42- is a spectator ion and is omitted from the ionic equation.
What is the formula of the phosphate ion?
PO43-
Phosphate is a polyatomic ion made up of one phosphorus atom and four oxygen atoms, carrying a 3- charge.
True or False?
In the combustion of organic compounds, it is recommended to balance carbon first, then hydrogen, then oxygen.
True.
Following this order avoids repeated re-balancing and makes the process systematic.
Define percentage yield.
A Percentage yield is calculated as: (actual yield / theoretical yield) × 100.\n\nIt compares the mass or moles of product actually obtained to the maximum amount predicted from stoichiometry.
The .......... reagent is the reactant that is completely consumed first. Once it is used up, the reaction .......... , even if other reactants remain.
The limiting reagent is the reactant that is completely consumed first. Once it is used up, the reaction stops, even if other reactants remain.
True or False?
Percentage yield can exceed 100 % if the reaction is very efficient.
False.
Percentage yield cannot exceed 100 %. Values above 100 % indicate experimental error, such as incomplete drying of a product or contamination.
What is an excess reagent?
An Excess reagent is a reactant that is present in a greater amount than needed to react completely with the limiting reagent.\n\nSome of the excess reagent remains unreacted when the reaction stops.
State three reasons why the actual yield of a reaction is often less than the theoretical yield.
The reaction does not go to completion.
Other reactions take place simultaneously, consuming reactants.
Reactants or products are lost to the atmosphere or during transfer.
9.2 g of sodium reacts with 8.0 g of sulfur: 2Na + S → Na2S.
Mol Na = .......... mol. Mol S = .......... mol. To react all Na requires .......... mol S. Since there is .......... mol S available, sulfur is in .......... .
Mol Na = 0.40 mol. Mol S = 0.25 mol. To react all Na requires 0.20 mol S. Since there is 0.25 mol S available, sulfur is in excess.
Sodium is therefore the limiting reagent.
True or False?
To identify the limiting reagent, you must take into account the molar ratio from the balanced equation, not just compare the raw number of moles.
True.
For example, in 2Na + S → Na2S, two moles of Na are needed for every one mole of S. Simply having more moles of Na than S does not mean Na is in excess.
Define atom economy.
An Atom economy is calculated as: (molecular mass of desired product / sum of molecular masses of all reactants) × 100.\n\nIt measures what fraction of the reactant atoms end up in the desired product.
True or False?
Addition reactions always have an atom economy of 100 %.
True.
In an addition reaction all reactant atoms are incorporated into a single product, so no atoms are wasted.
A high atom economy is desirable because it uses .......... raw materials, creates .......... waste and reduces the cost of waste .......... and disposal.
A high atom economy is desirable because it uses fewer raw materials, creates less waste and reduces the cost of waste separation and disposal.
Calculate the atom economy for the blast furnace reaction below, where iron is the desired product.
Fe2O3 + 3CO → 2Fe + 3CO2
(Ar: Fe = 55.8, O = 16.0, C = 12.0)
Sum of Mr of reactants = [2(55.8) + 3(16.0)] + 3[12.0 + 16.0] = 159.6 + 84.0 = 243.6
Mr of desired product (2Fe) = 2 × 55.8 = 111.6
Atom economy = (111.6 / 243.6) × 100 = 45.8 %
Ethanol can be made by hydration of ethene: C2H4 + H2O → C2H5OH.
The atom economy is .......... % because there is only .......... product and all atoms end up in the .......... product.
The atom economy is 100% because there is only one product and all atoms end up in the desired product.
True or False?
Atom economy and percentage yield measure the same thing.
False.
Atom economy is calculated from the balanced equation and shows how efficiently atoms are used regardless of how much product is actually collected. Percentage yield compares the actual mass collected to the theoretical maximum.
What is water of crystallisation?
A Water of crystallisation is water molecules that are chemically incorporated into the crystal structure of a compound.\n\nCompounds containing water of crystallisation are called hydrated salts and written with a dot, e.g. CuSO4·5H2O.
True or False?
Heating a hydrated salt converts it to the anhydrous form by removing water of crystallisation.
True.
For example: CuSO4·5H2O ⇌ CuSO4 + 5H2O. The reverse occurs when water is added back to the anhydrous salt.
Measure the mass of the .......... salt, heat until .......... mass, then calculate the mass of .......... lost. Use molar masses to find the .......... ratio of water to salt.
Measure the mass of the hydrated salt, heat until constant mass, then calculate the mass of water lost. Use molar masses to find the mole ratio of water to salt.
What is an anhydrous compound?
An Anhydrous is a term describing a compound that does not contain water of crystallisation.\n\nFor example, anhydrous copper(II) sulfate is CuSO4 — white in colour compared to the blue hydrated form CuSO4·5H2O.
11.25 g of CuSO4·xH2O is heated to constant mass of 7.19 g. What is the value of x?
Mass of water lost = 11.25 - 7.19 = 4.06 g
Mol CuSO4 = 7.19 / 159.5 = 0.045 mol
Mol H2O = 4.06 / 18 = 0.226 mol
Ratio = 0.226 / 0.045 = 5, so x = 5
Instead of elements, you start with the .......... and water. Instead of dividing by atomic masses, you divide by .......... masses.
Instead of elements, you start with the salt and water. Instead of dividing by atomic masses, you divide by molar (formula) masses.
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