S1.4 Counting particles by mass: The mole

Question 1

Mole

Answer 1

A mole is the SI unit for amount of substance, denoted as “n”. It represents a quantity of elementary entities such as atoms, molecules, ions, and electrons, with one mole containing exactly 6.02 × 10^23 of these entities, as defined by the Avogadro constant.

Question 2

What is the value of the Avogadro constant?

Answer 2

The Avogadro constant is 6.02 × 10^23 mol^−1. It is the number of elementary entities (atoms, molecules, ions, electrons) in one mole of a substance.

Question 3

How do you calculate the total number of elementary entities from moles?

Answer 3

Multiply the number of moles by the Avogadro constant (6.02 × 10^23 mol^−1) to find the total number of specified elementary entities (atoms, molecules, ions, electrons).

Question 4

What is relative atomic mass (Ar) and how is it measured?

Answer 4

Relative atomic mass (Ar) is the mass of an atom measured in comparison to the carbon-12 isotope. It represents how heavy an atom is relative to a carbon-12 atom, which is assigned a mass of 12 units.

Question 5

How do you calculate the relative formula mass (Mr) of a compound?

Answer 5

To calculate the relative formula mass (Mr) of a compound, sum up the relative atomic masses (Ar) of all the atoms in the compound’s formula as listed in the periodic table. This gives the mass of the compound relative to one twelfth of the mass of a carbon-12 atom.

Question 6

What does one mole represent in terms of the number of particles?

Answer 6

One mole represents Avogadro’s constant (6.022 x 10^23) of particles.

Question 7

Avogadro’s constant

Answer 7

Avogadro’s constant is 6.022 x 10^23 particles/mol, representing the number of atoms, ions, or molecules in one mole of substance.

Question 8

Molar mass (M)

Answer 8

The mass of one mole of a substance, expressed in units of g mol-1.

Question 9

How do you calculate the molar mass of a compound?

Answer 9

Sum the atomic masses of all atoms in the molecule according to the formula, expressed in grams per mole (g/mol).

Question 10

How to convert mass in grams to moles?

Answer 10

Divide the mass of the substance (in grams) by its molar mass (g/mol).

Question 11

How to convert moles to mass in grams?

Answer 11

Multiply the number of moles by the molar mass of the substance (g/mol).

Question 12

How to convert the number of particles to moles?

Answer 12

Divide the number of particles by Avogadro’s constant (6.022 x 10^23 particles/mol).

Question 13

How to convert moles to the number of particles?

Answer 13

Multiply the number of moles by Avogadro’s constant (6.022 x 10^23 particles/mol).

Question 14

1st step in interconverting the percentage composition by mass and the empirical formula of a compound?

Answer 14

Begin by calculating the molar mass of the compound.

Question 15

After calculating the molar mass, what is the next step in finding the percentage composition by mass of a compound?

Answer 15

Determine the mass contribution of each element to this molar mass.

Question 16

How do you find the percentage composition by mass of each element in a compound?

Answer 16

Divide the mass of each element by the molar mass of the compound and multiply by 100.

Question 17

How is the empirical formula derived from the percentage composition by mass?

Answer 17

Use these percentages to find the simplest whole number ratio of atoms, which gives the empirical formula.

Question 18

What is the first step in determining the molecular formula from the empirical formula and molar mass?

Answer 18

Calculate the empirical formula mass by adding the atomic masses of the elements in the empirical formula.

Question 19

How do you find the multiplier to convert the empirical formula to the molecular formula?

Answer 19

Divide the molar mass of the compound by the empirical formula mass to find a multiplier.

Question 20

Once you have the multiplier, how do you determine the molecular formula from the empirical formula?

Answer 20

Multiply the subscripts in the empirical formula by this multiplier to get the molecular formula.

Question 21

What are the qualitative descriptions for solution concentrations?

Answer 21

Solutions can be described as dilute (lower ratio of solute to solvent) or concentrated (higher ratio of solute to solvent).

Question 22

How do you calculate the molar concentration of a solution?

Answer 22

Use the formula C = n/V for moles per volume, where C is the concentration, n is the amount in moles, and V is the volume of the solution in liters.

Question 23

What equipment is required to accurately prepare a solution of known concentration?

Answer 23

A mass balance to measure the solute mass accurately, and a volumetric flask to measure the solution volume accurately.

Question 24

What factors influence the number of gas particles in a sealed container?

Answer 24

The factors include the container’s volume, the gas’s temperature, and the gas’s pressure. Larger volumes accommodate more particles, higher temperatures increase kinetic energy affecting capacity, and higher pressure means more particle collisions with container sides.

Question 25

What is Avogadro’s law regarding gases?

Answer 25

Avogadro’s law states equal volumes of all gases, at constant temperature and pressure, contain equal numbers of particles, implying volume is directly proportional to the number of molecules.

Question 26

How are mole ratios used for gaseous reactants and products?

Answer 26

Use the balanced equation’s mole ratio, leveraging Avogadro’s law that gas volumes translate into mole ratios. For instance, a reaction with 2 volumes of H₂ and 1 volume of O₂ producing 2 volumes of H₂O vapor shows direct mole ratio deduction based on gas volumes under constant conditions.