Calculations Involving Masses Flashcards

All 34 cards in this deck

What is relative formula mass ($M_r$)?

The sum of the relative atomic masses of all the atoms shown in the formula of a substance.

What is the equation for the percentage by mass of an element in a compound?

% by mass = (total $A_r$ of that element's atoms in the formula ÷ $M_r$ of the compound) × 100

True or false? When finding the percentage by mass of hydrogen in $\mathrm{NH_3}$, you use $3 \times 1.0$ as the mass of hydrogen.

True. You must use the mass of all the atoms of that element in the formula, not just one.

What is an empirical formula?

The formula showing the simplest whole number ratio of atoms of each element in a compound.

What is a molecular formula?

The formula showing the actual number of atoms of each element in one molecule of the substance.

How do you deduce the empirical formula from a molecular formula?

e.g. ethene, $\mathrm{C_2H_4}$

Divide all the subscripts by their highest common factor.

e.g. $\mathrm{C_2H_4}$ → $\mathrm{CH_2}$

How is the molecular formula related to the empirical formula and the $M_r$?

Multiply every number in the empirical formula by $M_r$ ÷ (relative mass of the empirical formula).

True or false? An ionic compound such as sodium chloride is normally written as its empirical formula.

True. Ionic compounds have giant lattices, so the formula gives the simplest ratio of ions.

When calculating an empirical formula, what do you divide each element's mass by?

Its relative atomic mass, to give the number of moles.

In the magnesium oxide empirical formula experiment, how do you show the magnesium has reacted completely?

Reheat and reweigh until the mass remains constant (heat to constant mass).

In the magnesium oxide experiment, why is the crucible lid lifted briefly during heating?

To let oxygen in while stopping magnesium oxide smoke escaping, which would make the product mass too low.

State the law of conservation of mass.

The total mass of the products equals the total mass of the reactants; no atoms are created or destroyed in a chemical reaction.

Why does the mass not change in a precipitation reaction carried out in a closed flask?

No gas enters or leaves, so all the atoms stay in the closed system.

Why does the mass decrease when a metal carbonate is heated in an open flask?

A gas (carbon dioxide) is given off and escapes from the non-enclosed system.

Why does the mass increase when a metal is heated in air in an open crucible?

The metal combines with oxygen from the air, which adds to the mass of the solid.

True or false? A decrease in mass during a reaction in an open flask means the law of conservation of mass has been broken.

False. Mass is still conserved — a gas has escaped into the surroundings.

In a balanced equation, what links the number of moles of a reactant and a product?

The ratio of the numbers in front of their formulae.

e.g. in $2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$, 2 mol Mg gives 2 mol MgO (a 1 : 1 ratio).

In a reacting mass calculation using a mass ratio, what units does the answer come out in?

The units of the mass given in the question, e.g. tonnes in gives tonnes out.

What does the mass ratio $160 : 112$ from $\mathrm{Fe_2O_3}$ to $2\mathrm{Fe}$ tell you?

160 tonnes of $\mathrm{Fe_2O_3}$ gives 112 tonnes of iron, so iron is $112/160 = 0.7$ of the oxide's mass.

What is the formula for the concentration of a solution in g dm⁻³?

concentration = mass of solute (g) ÷ volume of solution (dm³)

How do you find the mass of solute from a concentration in g dm⁻³ and a volume?

mass of solute (g) = concentration (g dm⁻³) × volume of solution (dm³)

How is one mole of a substance defined in terms of number of particles?

The Avogadro constant number of particles, $6.02 \times 10^{23}$ atoms, molecules, formulae or ions.

What is the mass of one mole of a substance?

Its relative particle mass in grams.

e.g. 1 mol of H atoms has a mass of 1.00 g

True or false? One mole of oxygen gas, $\mathrm{O_2}$, contains $6.02 \times 10^{23}$ oxygen atoms.

False. It contains $6.02 \times 10^{23}$ molecules, which is $1.204 \times 10^{24}$ atoms.

What is the formula linking moles, mass and relative formula mass?

moles = mass (g) ÷ relative formula mass

What is the equation linking the number of particles and the number of moles?

number of particles = moles × Avogadro constant ($6.02 \times 10^{23}$)

What is the limiting reactant in a reaction?

The reactant that is not in excess — it is completely used up and controls the mass of product formed.

Why does the limiting reactant control the mass of product formed?

Once it is all used up the reaction stops, so no more product can be made even though other reactants remain.

True or false? The reactant present in the smaller mass is always the limiting reactant.

False. You must compare moles using the ratio in the balanced equation, not masses.

What does the stoichiometry of a reaction mean?

The balancing numbers in the equation — the mole ratio in which the substances react and are formed.

When deducing the stoichiometry of a reaction from reacting masses, what gives the balancing numbers?

The simplest whole-number ratio of the moles of each substance, where moles = mass (g) ÷ relative mass ($A_r$ or $M_r$).

When finding moles of iron from a mass for an equation written as $2\mathrm{Fe}$, what relative mass must you divide by?

56 (the $A_r$ of one Fe atom), not 112.

How is the amount of product made related to the amount of limiting reactant used?

It is directly proportional: doubling the moles of limiting reactant doubles the moles of product (if the other reactants are in excess).

How do you convert a volume in cm³ into dm³?

Divide by 1000, because 1 dm³ = 1000 cm³.