5.1 The mole and Avogadro's constant

Question 1

What is stoichiometry?

Answer 1

Stoichiometry is the ratio of the reactants and products in a balanced chemical equation.

Question 2

What is a mole in chemistry?

Answer 2

A mole is a unit used to measure the amount of a substance. One mole contains 6.02 × 10²³ particles (Avogadro’s constant) and has a mass in grams equal to its relative formula mass.

Question 3

What is the Avogadro constant?

Answer 3

The Avogadro constant is 6.02 × 10²³, which is the number of atoms, molecules, or formula units in one mole of a substance.

Question 4

What is molar mass?

Answer 4

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol).

Question 5

How is molar mass calculated?

Answer 5

To calculate molar mass, add the relative atomic masses of all atoms in the formula. For example, the molar mass of ethanol (C₂H₆O) is: (2 × 12) + (6 × 1) + 16 = 46 g/mol

Question 6

How do you calculate the number of moles from mass?

Answer 6

Number of moles = Mass (g) / Molar Mass (g/mol)

Question 7

How do you calculate mass from moles?

Answer 7

Mass (g) = Number of moles × Molar Mass (g/mol)

Question 8

What is the empirical formula of a compound?

Answer 8

The empirical formula is the simplest whole number ratio of atoms in a compound.

Question 9

What is the molecular formula of a compound?

Answer 9

The molecular formula shows the actual number of each type of atom in a molecule. It may be a multiple of the empirical formula.

Question 10

How do you determine the molecular formula from the empirical formula?

Answer 10

1. Calculate the relative formula mass (RFM) of the empirical formula. 2. Divide the actual molecular mass by the RFM of the empirical formula. 3. Multiply the subscripts in the empirical formula by this number.

Question 11

What is a hydrated salt?

Answer 11

A hydrated salt is a salt that contains water molecules as part of its crystal structure.

Question 12

How do you find the formula of a hydrated salt?

Answer 12

1. Heat the salt to remove the water. 2. Subtract to find the mass of water lost. 3. Calculate the moles of salt and water. 4. Find the simplest ratio and write the formula as: salt·xH₂O

Question 13

Example: Hydrated Copper(II) sulfate
If 5.0 g of CuSO₄·xH₂O gives 3.2 g of CuSO₄ after heating, what is the mass of water lost?

Answer 13

5.0 g - 3.2 g = 1.8 g of water lost