Formulae & Reacting Quantities (SQA National 5 Chemistry): Flashcards
Exam code: X813 75
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What does the –ide ending in a compound name usually tell you?
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What does the –ide ending in a compound name usually tell you?
The –ide ending usually tells you the compound contains only two elements. The non-metal's name is changed to end in –ide (e.g. chlorine becomes chloride, sulfur becomes sulfide).
True or False?
Sodium hydroxide (NaOH) is an exception to the –ide rule because it contains three elements.
True.
Sodium hydroxide contains sodium, oxygen and hydrogen. The hydroxide ion (OH-) contains two elements, so –ide compounds can sometimes contain three elements.
What does the –ate or –ite ending in a compound name tell you about the elements present?
The –ate or –ite ending tells you that oxygen is present in addition to the other named elements. These compounds typically contain group ions (polyatomic ions) made of more than one atom.
Copper carbonate (CuCO3) ends in –ate, which means it contains copper, carbon and ...........
Copper carbonate (CuCO3) ends in –ate, which means it contains copper, carbon and oxygen.
True or False?
The sulfate ion (SO42-) contains more oxygen atoms than the sulfite ion (SO32-).
True.
Sulfate is SO42- (four oxygen atoms) and sulfite is SO32- (three oxygen atoms). The –ate ending always indicates more oxygen than the corresponding –ite compound.
State the three key name endings used to identify elements in chemical compounds.
The three key endings are:
–ide (usually two elements)
–ite (oxygen present plus other named elements)
–ate (oxygen present plus other named elements; more oxygen than –ite)
The compound potassium nitrate ends in .........., which tells us that nitrogen and oxygen are both present alongside potassium.
The compound potassium nitrate ends in –ate, which tells us that nitrogen and oxygen are both present alongside potassium.
How does the name of a non-metal element change when it becomes an ion in a compound?
How the non-metal name changes: the ending is replaced with –ide. For example, chlorine becomes chloride, sulfur becomes sulfide, oxygen becomes oxide and bromine becomes bromide.
True or False?
Iron sulfide (FeS) contains three elements.
False.
Iron sulfide contains only two elements: iron (Fe) and sulfur (S). The –ide ending indicates a two-element compound (with exceptions such as hydroxides).
Name the elements present in calcium phosphate.
Calcium phosphate contains three elements: calcium, phosphorus and oxygen. The –ate ending confirms that oxygen is present alongside the other named elements.
Define valency.
Valency is the combining power of an atom — the number of chemical bonds it can form. For Groups 1–4, valency equals the group number; for Groups 5–7, valency equals (8 minus the group number).
What is the valency of an element in Group 6 of the Periodic Table?
The valency of a Group 6 element is 2. This is calculated as 8 minus the group number: 8 – 6 = 2.
True or False?
The prefix di– in a compound name means there are two atoms of that element in one molecule.
True.
The prefix di– indicates two atoms. For example, carbon dioxide (CO2) has two oxygen atoms. Prefixes are used only for covalent compounds, not ionic ones.
In iron(III) oxide, the Roman numeral (III) tells you that the iron ion has a charge of ...........
In iron(III) oxide, the Roman numeral (III) tells you that the iron ion has a charge of 3+.
Why are Roman numerals used in the names of some transition metal compounds?
Roman numerals are used because transition metals can form more than one type of ion with different charges. The Roman numeral states the exact valency (and therefore charge) of the metal in that specific compound, avoiding confusion.
What is the swap-and-drop method?
The swap-and-drop method is a technique for writing ionic formulae. The size of the charge on each ion is swapped to become the subscript number of the other ion, then the positive and negative signs are dropped.
True or False?
The formula for a covalent molecular substance shows the simplest ratio of atoms of each element in the molecule.
False.
The formula of a covalent molecular substance shows the exact number of atoms in one molecule. It is covalent network substances that show the simplest whole-number ratio of atoms.
The formula SiO2 represents silicon dioxide, a covalent network substance. This means for every 1 silicon atom there are .......... oxygen atoms in the giant lattice.
The formula SiO2 represents silicon dioxide, a covalent network substance. This means for every 1 silicon atom there are 2 oxygen atoms in the giant lattice.
How do you find the valency of a group ion such as sulfate (SO42-)?
The valency of a group ion is the number in its charge. Sulfate has a charge of 2–, so its valency is 2. This can be looked up in the SQA Data Booklet (page 8).
What is the formula of copper(II) chloride, and how is it determined?
The formula of copper(II) chloride is CuCl2. Copper(II) has a charge of 2+ and chloride has a charge of 1–. Using the swap-and-drop method, 2 becomes the subscript on chloride and 1 (omitted) on copper, giving CuCl2.
Define a word equation in chemistry.
A word equation shows the reactants and products of a chemical reaction using their full chemical names, with reactants on the left side of an arrow and products on the right.
Why must a symbol equation be balanced?
A symbol equation must be balanced because of the law of conservation of mass — atoms cannot be created or destroyed in a chemical reaction, so the number of each type of atom must be the same on both sides of the equation.
When balancing equations, you may only change the .......... in front of a formula — never alter the small numbers within the formula itself.
When balancing equations, you may only change the coefficients in front of a formula — never alter the small numbers within the formula itself.
True or False?
Non-metal elements such as hydrogen, oxygen and chlorine exist as diatomic molecules.
True.
Several non-metals exist as diatomic molecules and must be written as H2, O2, N2, F2, Cl2, Br2 and I2 in symbol equations.
State the four state symbols used in chemical equations and what each represents.
The four state symbols are:
(s) — solid
(l) — liquid
(g) — gas
(aq) — aqueous (dissolved in water)
To balance 2Al + .......... CuO → Al2O3 + 3Cu, the missing coefficient is 3 because three oxygen atoms are needed to match the .......... in Al2O3.
To balance 2Al + 3 CuO → Al2O3 + 3Cu, the missing coefficient is 3 because three oxygen atoms are needed to match the three oxygen atoms in Al2O3.
True or False?
A reaction condition or catalyst can be written above the arrow in a chemical equation.
True.
Conditions such as temperature or pressure, and catalyst names, are written above the reaction arrow to provide extra information without changing the equation itself.
How should you treat a group of atoms like NO3- when balancing a symbol equation?
When the group ion appears unchanged on both sides of the equation, treat the whole group (e.g. NO3-) as a single entity rather than counting individual atoms. This makes balancing faster and reduces errors.
What is the balanced symbol equation for the combustion of sulfur in oxygen?
The balanced equation is:
S (s) + O2 (g) → SO2 (g)
One sulfur atom reacts with one oxygen molecule to produce one molecule of sulfur dioxide. The equation is already balanced with no coefficients needed.
Define gram formula mass (GFM).
Gram formula mass (GFM) is the mass of one mole of a substance, measured in grams. It is calculated by adding the relative atomic masses (RAMs) of all atoms in the chemical formula.
What is the GFM of magnesium bromide, MgBr2?
The GFM of MgBr2 is 184.5 g.
GFM = (1 × 24.5) + (2 × 80) = 24.5 + 160 = 184.5 g
The formula for calculating the number of moles is: n = mass ÷ ...........
The formula for calculating the number of moles is: n = mass ÷ GFM.
True or False?
The relative atomic masses needed for GFM calculations are found on the periodic table.
True.
Page 7 of the SQA Data Booklet provides relative atomic masses (RAMs) for all elements, which are needed to calculate the GFM of any substance.
How many moles are in 8 g of methane, CH4?
There are 0.5 moles in 8 g of methane.
GFM of CH4 = 12 + (4 × 1) = 16 g
n = mass ÷ GFM = 8 ÷ 16 = 0.5 moles
What is a reacting mass calculation?
A reacting mass calculation uses the mole ratio from a balanced chemical equation to find the mass of a reactant or product when the mass of another substance in the reaction is known.
In a reacting mass calculation, after finding moles of the known substance, you use the .......... from the balanced equation to find moles of the unknown substance.
In a reacting mass calculation, after finding moles of the known substance, you use the mole ratio from the balanced equation to find moles of the unknown substance.
True or False?
In a multi-step mole calculation, you should round intermediate answers to 2 significant figures.
False.
Rounding intermediate answers introduces rounding errors that can make the final answer inaccurate and cost marks. Only round the final answer.
State the three steps used to solve a reacting mass calculation.
The three steps are:
Calculate moles of the known substance (n = mass ÷ GFM)
Use the mole ratio from the balanced equation to find moles of the unknown
Calculate mass of the unknown (mass = n × GFM)
What is the mass of 0.5 moles of carbon dioxide, CO2?
The mass is 22 g.
GFM of CO2 = 12 + (2 × 16) = 44 g
mass = n × GFM = 0.5 × 44 = 22 g
Define concentration of a solution.
Concentration is a measure of how much solute is dissolved in a given volume of solvent. It is measured in moles per litre (mol l-1).
What is the relationship between moles, concentration and volume in a solution?
The relationship is: n = c × v, where n is moles, c is concentration in mol l-1, and v is volume in litres. Rearranged: c = n / v and v = n / c.
Before using the formula n = c × v, volume given in cm³ must be divided by .......... to convert it to litres.
Before using the formula n = c × v, volume given in cm³ must be divided by 1000 to convert it to litres.
True or False?
A solution with a high concentration contains a small amount of solute dissolved in a large volume of solvent.
False.
A high concentration means there is a large amount of solute dissolved. A low (dilute) concentration means there is very little solute present.
Calculate the concentration of a solution made by dissolving 10 g of sodium hydroxide (NaOH) in 250 cm³ of water.
The concentration is 1 mol l-1.
GFM of NaOH = 23 + 16 + 1 = 40 g
n = 10 ÷ 40 = 0.25 mol
v = 250 ÷ 1000 = 0.25 l
c = n ÷ v = 0.25 ÷ 0.25 = 1 mol l-1
The solute is the substance that .........., and the solvent is the liquid it dissolves in.
The solute is the substance that dissolves, and the solvent is the liquid it dissolves in.
True or False?
Concentration can be expressed in units of mol/l or mol l-1.
True.
Both mol/l and mol l-1 are correct ways to write the units of concentration. They mean the same thing: moles of solute per litre of solution.
What mass of potassium chloride (KCl) is needed to make 500 cm³ of a 0.2 mol l-1 solution?
The mass needed is 7.45 g.
v = 500 ÷ 1000 = 0.5 l
n = c × v = 0.2 × 0.5 = 0.1 mol
GFM of KCl = 39 + 35.5 = 74.5 g
mass = 0.1 × 74.5 = 7.45 g
How do you find the volume of solution needed if you know the moles of solute and the concentration?
Rearrange n = c × v to give v = n ÷ c. Divide the number of moles by the concentration (in mol l-1) to get the volume in litres.
Define percentage composition.
Percentage composition expresses the mass of each element in a compound as a percentage of the total mass (the GFM) of the compound.
What is the formula used to calculate the percentage composition of an element in a compound?
The formula is:
Percentage composition = (total mass of element ÷ GFM) × 100
The total mass of the element accounts for all atoms of that element in the formula.
To calculate percentage composition, you first find the GFM of the compound, then divide the total .......... of the element by the GFM and multiply by 100.
To calculate percentage composition, you first find the GFM of the compound, then divide the total mass of the element by the GFM and multiply by 100.
True or False?
The percentage compositions of all elements in a compound should add up to 100%.
True.
Because percentage composition accounts for every element in the compound by mass, the sum of all individual percentages must equal 100%.
Calculate the percentage composition of carbon in glucose, C6H12O6.
The percentage of carbon in glucose is 40%.
GFM = (6×12) + (12×1) + (6×16) = 72 + 12 + 96 = 180 g
Percentage of C = (6 × 12 ÷ 180) × 100 = 40%
Ammonium nitrate has the formula NH4NO3. It contains .......... nitrogen atoms in total, which must both be included in the percentage composition calculation.
Ammonium nitrate has the formula NH4NO3. It contains two nitrogen atoms in total, which must both be included in the percentage composition calculation.
True or False?
A percentage composition value greater than 100% is possible if the experiment is carried out carefully.
False.
A value above 100% is never possible and always indicates an error. The most common cause is placing GFM in the numerator and element mass in the denominator — the fraction is upside down.
Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3.
The percentage of nitrogen in ammonium nitrate is 35%.
GFM = 14 + 4 + 14 + 48 = 80 g
Total mass of N = 2 × 14 = 28 g
Percentage = (28 ÷ 80) × 100 = 35%
How does percentage composition help assess the value of a fertiliser?
Percentage composition tells you how much of a useful element (e.g. nitrogen) is present per unit mass of fertiliser. A higher percentage of nitrogen means more nutritional value per gram, so it is more cost-effective.
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