12th grade chemical reactions, acids and bases

Example

Find the empirical formula: 11.2 g of iron combines with 4.8 g of oxygen (Ar: Fe = 56, O = 16).

1. Fe 0.2 : O 0.3 = 2 : 3
2. So the answer is: Fe₂O₃

Answer: Fe₂O₃

Example

CH₄ + 2O₂ → CO₂ + 2H₂O. 1 mol of methane is mixed with 1 mol of oxygen. Which is the limiting reactant?

1. methane: 1 ÷ 1 = 1; oxygen: 1 ÷ 2 = 0.5
2. So the answer is: oxygen

Answer: oxygen

Example

20 cm³ of 0.5 mol/dm³ sodium hydroxide is neutralised by 12.5 cm³ of hydrochloric acid. HCl + NaOH → NaCl + H₂O. What is the concentration of the acid, in mol/dm³? Give 3 decimal places.

1. n(NaOH) = 0.5 × 0.02 = 0.01
2. n(HCl) = 0.01
3. 0.01 ÷ 0.0125 = 0.8
4. So the answer is: 0.8 mol/dm³

Answer: 0.8 mol/dm³

Example

10 cm³ of 1 mol/dm³ acid is diluted with water to a total volume of 20 cm³. What is the new concentration, in mol/dm³?

1. 1 × 10 ÷ 20 = 0.5
2. So the answer is: 0.5 mol/dm³

Answer: 0.5 mol/dm³

Example

Complete the half-equation at the cathode: Cu²⁺ + ? e⁻ → Cu. How many electrons are needed?

1. Cu²⁺ needs 2 electron(s) to become a neutral copper atom.
2. So the answer is: 2

Answer: 2

Example

Solution X has pH 5 and solution Y has pH 7. How many times greater is the hydrogen ion concentration in X than in Y?

1. 10^2 = 100
2. So the answer is: 100

Answer: 100