1.6 Calculations Involving Masses (Edexcel GCSE Chemistry)

The molecular formula of ethanoic acid is C₂H₄O₂.

Empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit.

The empirical formula of propene is CH₂.

False.

Organic molecules often have different empirical and molecular formulae.

The formula of an ionic compound is always an empirical formula.

The empirical formula is calculated from the ratio of masses of each element in the compound.

The molecular formula is a multiple of the empirical formula.

It indicates the exact numbers of each atom in the compound.

False.

Empirical formula shows the simplest whole number ratio of atoms in a molecule.

To calculate the molecular formula from the empirical formula:

1. Calculate the relative formula mass of the empirical formula.

2. Divide the relative formula mass of the compound by the mass of the empirical formula.

3. Multiply each number of elements by this number to find the molecular formula.

False.

The empirical formula of the compound is C₂H₅Br.

Empirical formula = CH₂Cl

Molecular formula = C₂H₄Cl₂

The empirical formula and molecular fomula of propanoic acid are both C₃H₆O₂.

False.

The empirical formula of ethanoic acid is CH₂O, while the molecular formula is C₂H₄O₂.

The two mass measurements you need to know to determine the empirical formula of a metal oxide are:

• The initial mass of the metal

• The final mass of the metal oxide

The element that reacts with a metal to form a metal oxide is oxygen.

The information from the Periodic Table that is required to calculate the empirical formula of a metal oxide is:

• The atomic mass of the metal

• The atomic mass of oxygen

The equation to calculate the moles of metal and oxygen in a metal oxide is:

moles = mass atomic mass (A_r)

A lid is needed to reduce / prevent the loss of the metal oxide / product.

The crucible lid is frequently lifted during an empirical formula experiment to allow air / oxygen in for the reaction.

The mass of magnesium is 25.4 - 24.6 = 0.8 g

The mass of magnesium oxide is 25.9 - 24.6 = 1.3 g

The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.

True.

The total mass of reactants is always equal to the total mass of products.

A precipitation reaction is a reaction where two solutions react to form an insoluble solid called a precipitate.

In a closed system precipitation reaction, the total mass remains constant.

In an open system when a gaseous product is formed, the total mass of the reaction flask will decrease as the gas escapes.

False.

Mass cannot be created during a chemical reaction. The Law of Conservation of Mass states that mass is conserved.

The mass of a reaction flask might increase if one of the reactants is a gas found in the air and all of the products are either solids or liquids.

A precipitate is an insoluble solid that forms during a chemical reaction.

False.

The Law of Conservation of Mass applies to all chemical reactions, not just reactions in solution.

During a chemical reaction:

• The overall mass of the reactants and products remains constant / the same.

• Atoms are rearranged as reactants form new chemicals / products.

False.

When completing reacting masses calculations, the mass unit (e.g. grams, tonnes) does not affect the calculation.

The mass unit only affects the final anwer as it must be given in the appropriate units.

The correct order of steps to complete a reacting mass calculation is:

1. Write a balanced symbol equation.

2. Calculate relevant molar masses.

3. Calculate the moles of the known chemical.

4. Check the molar ratio in the equation.

5. Determine the moles of the target chemical.

6. Calculate the mass of the target chemical.

CuCO₃ CuO + CO₂

The molar ratio of copper carbonate to copper oxide is 1 : 1.

2Na + Cl₂ 2NaCl

The molar ratio of chlorine to sodium chloride is 1 : 2.

False.

The masses of reactants and products in a reaction can be used to write a balanced equation for the reaction.

How the masses of the reactants and products be used to form a balanced chemical equation:

• Convert all masses to moles

• Find the molar ratio of all chemicals

• Simplify the molar ratio

• Place these values in front of each chemical in the equation

False.

3A + B 2C + 2D

The molar ratio of A : C is not 2 : 3.

There are 3 moles of A and 2 moles of C, which means the ratio is 3 : 2.

The moles, mass, molar mass equation that is needed for reacting mass questions is:

moles = mass / molar mass

The equation for the percentage by mass of an element is:

A solute is a solid substance that dissolves in a liquid / solvent.

To convert from dm³ to cm³, multiply by 1000.

A solvent is the liquid in which a solid / solute dissolves.

Concentration refers to the amount of solid / solute there is in a specific volume of the liquid / solvent.

A solution is the mixture formed when a solid / solute dissolves in a liquid / solvent.

The equation to calculate concentration is:

Concentration = Amount of solute / Volume of solution.

False.

1 dm³ is equal to 1000 cm³, not 100 cm³.

Concentration is typically measured in grams per cubic decimetre (g/dm³).

To convert from cm³ to dm³, divide by 1000.

False.

A higher concentration means more solute in a given volume, not less.

The total number of ions in one mole of NaCl is:

2 x 6.02 x 10²³ = 1.204 x 10²⁴

Using moles, mass in grams and the molar mass, the equation is:

Moles = mass / molar mass

True.

There are 6.02 x 10²³ carbon dioxide molecules in one mole of carbon dioxide.

The equation for molar mass using moles and mass is:

molar mass = mass / moles

To calculate the number of moles:

• Moles = mass / molar mass

• Moles = 45.0 / 18.0

• Moles = 2.5

To calculate the mass:

• Mass = moles x molar mass

• Mass = 0.25 x 123.5

• Mass = 30.9 (or 30.875) g

To calculate the molar mass:

Molar mass = mass / moles

Molar mass = 22.0 / 0.5

Molar mass = 44.0 g mol^-1

To calculate the molar mass of a compound for a moles, mass, molar mass calculation, you add the atomic masses of all the atoms in the compound.

The equation connecting moles, mass and molar mass is:

mass = moles x molar mass

The limiting reagent is the reactant that is used up first in a chemical reaction, limiting the amount of product that can be formed.

The amount of product formed in a reaction is determined by the limiting reagent.

False.

The limiting reagent is not always the reactant with the smallest mass. It depends on the stoichiometry of the reaction.

A reactant is in excess when it is present in an amount greater than necessary to react with the limiting reagent.

The steps to determine the limiting reagent are:

1. Write the balanced equation for the reaction.

2. Calculate the moles of each reactant.

3. Compare the moles & deduce the limiting reactant.

False.

There can only be one limiting reagent in a reaction.

After the reaction is complete, the reactants in excess remain unreacted.

The limiting reagent determines the maximum possible yield of a reaction.

True.

The limiting reagent is always completely consumed in a reaction.

An easy way to determine the limiting reactant is:

1. Find the moles of each substance.

2. Divide by the coefficient in the equation.

3. The lowest resulting number is the limiting reactant.

True.

A balanced chemical equation has the same number of atoms of each element on both sides, in accordance with the Law of Conservation of Mass.

The word equation for the reaction of silicon with oxygen to form silicon dioxide is:

silicon + oxygen silicon dioxide

2Br^- + Cl₂ Br₂ + 2Cl^- is an ionic equation.

A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction.

The missing numbers to balance the equation are:

2H₂O₂ 2H₂O + O₂

Products are the new substances which are on the right-hand side of the arrow in a chemical equation, formed by the reaction of reactants.

The constituent ions are:

• H₂SO₄ - two hydrogen ions (H⁺) and one sulfate ion (SO₄^2-)

• NaOH - one sodium ion (Na⁺) and one hydroxide ion (OH^-)

• KBr - one potassium ion (K⁺) and one bromide ion (Br^-)

The symbol equation for the reaction of magnesium with chlorine to form magnesium chloride is:

Mg + Cl₂ MgCl₂

False.

The balanced symbol equation for the reaction of sodium with chlorine is:

2Na + Cl₂ 2NaCl

The missing numbers to balance the equation are:

N₂ + 3H₂ 2NH₃

Spectator ions are ions which appear on both sides of an ionic equation and are eliminated from the final equation.

2KI + Br₂ 2KBr + I₂ is not an ionic equation because there are no ions.