2.3 AS Level

Formulas

10 flashcards to master this topic

Definition Flip

Write the formula for aluminum oxide.

Answer Flip

Aluminum has a charge of +3 (Al³⁺) and oxygen has a charge of -2 (O²⁻). To balance the charges, the formula is Al₂O₃.

Definition Flip

Write the formula for copper(II) sulfate.

Answer Flip

Copper(II) indicates a +2 charge (Cu²⁺). Sulfate is SO₄²⁻. Therefore, the formula is CuSO₄.

Definition Flip

Define empirical formula.

Answer Flip

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

Example: The empirical formula of glucose (C₆H₁₂O₆) is CH₂O.
Definition Flip

Define molecular formula.

Answer Flip

The molecular formula is the actual number of atoms of each element in a molecule.

Example: The molecular formula of glucose is C₆H₁₂O₆.
Definition Flip

Define anhydrous.

Answer Flip

Anhydrous means a substance contains no water molecules. It is the opposite of hydrated.

Example: Anhydrous CuSO₄ is white.
Definition Flip

Define hydrated.

Answer Flip

Hydrated means a substance contains water molecules within its crystal structure.

Example: Hydrated copper(II) sulfate (CuSO₄•5H₂O) is blue.
Definition Flip

Define water of crystallisation.

Answer Flip

Water of crystallisation refers to water molecules that are chemically bonded within the crystal structure of a hydrated compound.

Example: In CuSO₄•5H₂O, the '5H₂O' represents water of crystallisation.
Calculation Flip

A compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Calculate its empirical formula.

Answer Flip

Assume 100g, so 40g C, 6.7g H, 53.3g O. Convert to moles: C = 40/12 = 3.33, H = 6.7/1 = 6.7, O = 53.3/16 = 3.33. Divide by smallest (3.33): C = 1, H = 2, O = 1. Empirical formula is CH₂O.

Calculation Flip

0.100g of magnesium reacts with excess hydrochloric acid to produce 0.00821 mol of hydrogen gas. What is the percentage yield of hydrogen?

Answer Flip

Mg + 2HCl → MgCl₂ + H₂. Moles of Mg = 0.100/24.3 = 0.00411 mol. Theoretical yield of H₂ = 0.00411 mol. % yield = (actual yield / theoretical yield) x 100 = (0.00821 / 0.00411) x 100 = 200%. Note, must be less than 100%. Mistake in question, 0.002055 mol of hydrogen gas is correct. Actual yield = 0.002055 % yield = 50%

Calculation Flip

25.0 cm³ of 0.100 mol dm⁻³ NaOH is neutralised by 20.0 cm³ of HCl. What is the concentration of the HCl?

Answer Flip

NaOH + HCl → NaCl + H₂O. Moles of NaOH = (25/1000) * 0.1 = 0.0025 mol. Moles of HCl = moles of NaOH = 0.0025 mol. Concentration of HCl = moles/volume = 0.0025/(20/1000) = 0.125 mol dm⁻³

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2.2 The mole and the Avogadro constant 3.1 Electronegativity and bonding