2.2 Amount of Substance for A-Level Chemistry (OCR)

What is the weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom?

relative atomic mass

Symbol Ar. Takes isotopic abundance into account.

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Relative atomic mass is measured on a scale where which isotope has a value of exactly 12?

carbon-12

The 12C isotope is the standard reference for atomic masses.

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What is the sum of the relative atomic masses of all the atoms in a molecule called?

relative molecular mass

Symbol Mr. Used for covalent substances.

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What term is used instead of relative molecular mass when referring to ionic compounds?

relative formula mass

Ionic compounds do not exist as discrete molecules.

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True or False:

Relative molecular mass has units.

False

It is a relative quantity with no units.

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Why does relative atomic mass often differ from the mass number of the most common isotope?

isotopic abundances

Ar is a weighted average of all naturally occurring isotopes.

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What quantity would you calculate to find the Mr of CO₂?

relative molecular mass

Mr = Ar(C) + 2 × Ar(O).

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True or False:

Relative formula mass is calculated in the same way as relative molecular mass.

True

It is still the sum of relative atomic masses in the formula.

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What is the name given to the amount of substance that contains a fixed number of particles?

mole

The mole links microscopic particles to measurable quantities.

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What is the term for the number of particles in one mole of a substance?

Avogadro constant

Approximately 6.02 × 10²³ particles per mole.

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Fill in the blank:

One mole of any substance contains _______ × 10²³ particles.

6.02

This is the value of the Avogadro constant.

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What types of particles can the mole be used to count in chemistry?

electrons
atoms
molecules
ions

The mole applies to any specified particles.

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True or False:

One mole of different substances always contains different numbers of particles.

False

One mole always contains the same number of particles.

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What is the relationship used to calculate the amount of substance from mass and relative molecular mass?

n = m/Mr

n is moles, m is mass in grams.

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Fill in the blank:

The amount of substance (n) equals mass divided by _______.

Mass must be in grams.

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What unit is used to measure the concentration of a solution in chemistry?

mol dm⁻³

Often written as mol dm⁻³ or M.

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What is the relationship between concentration, amount of substance and volume of solution?

c = n/V

Volume must be in dm³.

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Fill in the blank:

Concentration equals amount of substance divided by _______.

volume

Volume must be converted to dm³ if given in cm³.

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True or False:

The mole can be applied to electrons in chemical calculations.

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True

The mole counts any specified particles.

What quantity links the mass of a substance to the number of moles present?

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relative molecular mass

Used in the equation n = m/Mr.

What is the simplest whole number ratio of atoms of each element in a compound called?

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empirical formula

Shows the simplest ratio, not the actual number of atoms.

What describes the actual number of atoms of each element present in a molecule?

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molecular formula

Represents the true composition of the molecule.

Fill in the blank:

The empirical formula shows the _______ whole number ratio of atoms in a compound.

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simplest

It may differ from the molecular formula.

True or False:

The molecular formula always matches the empirical formula.

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False

The molecular formula can be a multiple of the empirical formula.

What information must be known in addition to the **empirical formula** to determine the **molecular formula**?

relative molecular mass ## Footnote Used to determine the multiplication factor.

# Fill in the blank: The **molecular formula** equals the empirical formula multiplied by a _\_\_\_\_\_\_ _\_\_\_\_\_\_.

whole number ## Footnote Determined using Mr ÷ empirical formula mass.

What is the **first step** when calculating an empirical formula from percentage composition?

Convert masses to moles. ## Footnote Divide each mass by the relative atomic mass.

After calculating **moles of each element** in empirical formula calculations, what is the **next step**?

divide by smallest value ## Footnote This gives the simplest mole ratio.

What type of **compound** are students commonly asked to analyse when determining **empirical formula experimentally**?

metal oxide ## Footnote Oxygen mass can be found from mass change.

# True or False: Empirical formula calculations are based on **mole ratios of elements**.

True ## Footnote The mole ratio gives the simplest atomic ratio.

What **calculation** is used to determine the factor linking **empirical and molecular** formulas?

Mr divided by empirical formula mass ## Footnote The result should be a whole number multiplier.

What must be conserved when writing a **balanced chemical equation**?

number of atoms ## Footnote Atoms cannot be created or destroyed in a chemical reaction.

What type of **equation** shows only the **ions** that take part in a reaction?

ionic equation ## Footnote Spectator ions are omitted.

# Fill in the blanks: A **balanced equation** shows the correct _\_\_\_\_\_\_ _\_\_\_\_\_\_ of **reactants and products**.

mole ratios ## Footnote Coefficients represent mole relationships.

# True or False: The coefficients in a balanced equation represent the **ratio of moles reacting**.

True ## Footnote They allow stoichiometric calculations.

What is the **percentage of reactant atoms** that end up in the desired product called?

atom economy ## Footnote Indicates efficiency of a chemical process.

# Fill in the blank: **Atom economy** is calculated using the _\_\_\_\_\_\_ mass of the desired product divided by the sum of the masses of reactants.

molecular ## Footnote The value is multiplied by 100 to give a percentage.

Why is a **high atom economy** desirable in **industrial processes**?

less waste ## Footnote It improves sustainability and reduces costs.

# True or False: A reaction with a **high atom economy** always produces a **high yield**.

False ## Footnote Yield depends on reaction efficiency and losses.

What is the **percentage of the theoretical amount of product** that is actually obtained called?

percentage yield ## Footnote Often less than 100% due to losses or side reactions.

# Fill in the blank: **Percentage yield** equals actual yield divided by _\_\_\_\_\_\_ yield multiplied by 100.

theoretical ## Footnote Theoretical yield is calculated from stoichiometry.

What quantity can be calculated from a **balanced equation** when **masses of reactants** are known?

moles ## Footnote Stoichiometric ratios allow conversion between substances.

What **gas measurement** can be calculated using **balanced equations** when reactions involve gases?

volumes of gases ## Footnote Gas volumes follow mole ratios at the same conditions.

What two **quantities** are commonly used together to calculate **moles in solution reactions**?

concentration and volume ## Footnote Used with the equation n = cV.

# Fill in the blank: When calculating **moles from concentration**, the **volume** must be in _\_\_\_\_\_\_.

dm³ ## Footnote Convert from cm³ by dividing by 1000.

What type of **calculation** uses titres obtained during **titration experiments**?

concentration calculations ## Footnote Used to determine unknown concentrations.

# True or False: **Balanced equations** can be used to calculate masses, gas volumes, yields and concentrations.

True ## Footnote Stoichiometry links all quantities through mole ratios.

2.2 Amount of Substance Flashcards

Apply mole calculations to determine quantities, concentrations, and reacting masses in chemical systems. (47 cards)