1e-Chemical formulae, equations and calculations

Question 1

Question: What is a word equation?

Answer 1

Answer: A word equation shows the reactants and products of a chemical reaction using their names. Example: Hydrogen + Oxygen → Water

Question 2

Question: What is a balanced chemical equation?

Answer 2

Answer: A balanced equation shows the number of atoms of each element is the same on both sides. Example: 2H₂ + O₂ → 2H₂O

Question 3

Question: Why are state symbols used in equations?

Answer 3

Answer: State symbols show the physical state of substances: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

Question 4

Question: How do you calculate relative formula mass (Mr)?

Answer 4

Answer: Mr is the sum of the relative atomic masses (Ar) of all atoms in a formula. Example: Mr of H₂O = (2 × 1) + 16 = 18

Question 5

Question: What is a mole (mol)?

Answer 5

Answer: A mole is the unit of the amount of substance, containing 6.022 × 10²³ particles (Avogadro’s number).

Question 6

Question: How do you calculate the number of moles?

Answer 6

Answer: Moles = Mass / Mr

Question 7

Question: How do you calculate the mass of a substance?

Answer 7

Answer: Mass = Moles × Mr

Question 8

Question: What is percentage yield, and how is it calculated?

Answer 8

Answer: Percentage yield compares the actual and theoretical yield:Percentage yield = (Actual yield / Theoretical yield) × 100

Question 9

Question: What is an empirical formula?

Answer 9

Answer: The simplest ratio of atoms of each element in a compound.

Question 10

Question: What is a molecular formula?

Answer 10

Answer: The actual number of atoms of each element in a compound.

Question 11

Question: How do you calculate the empirical formula from experimental data?

Answer 11

Answer:

Find the mass of each element.

Calculate the moles of each element (Mass / Ar).

Divide by the smallest number of moles.

Write the ratio as the empirical formula.

Question 12

Question: How do you determine the formula of a metal oxide by combustion?

Answer 12

1. Weigh the metal before heating.
   
   2. Burn it in oxygen and weigh the metal oxide formed.
   3. Calculate mass of oxygen:
       mass of oxide – mass of metal
   4. Convert metal and oxygen masses to moles.
   5. Find the simplest whole number ratio of metal : oxygen.
   6. Write the empirical formula of the metal oxide.

Question 13

Question: How do you determine the formula of copper(II) oxide by reduction?

Answer 13

1. Weigh the copper(II) oxide before heating.
   
   2. Heat it with hydrogen gas (or methane) to reduce it to copper.
   3. Weigh the copper after the reaction.
   4. Calculate mass of oxygen removed:
       mass of copper(II) oxide – mass of copper
   5. Convert masses of copper and oxygen to moles.
   6. Find the simplest whole number ratio of copper : oxygen.
   7. Write the empirical formula of copper(II) oxide.