Edexcel International A Level Chemistry: Formulae & Equations

Formulae & Mass

Definitions

Understanding chemical formulae and mass is foundational in Chemistry. Here are essential terms:

Atom: The basic unit of a chemical element, the smallest particle that can take part in a chemical reaction.
Element: A pure substance that cannot be broken down into simpler substances by chemical means, representing a single type of atom.
Ion: An atom or molecule that has a net electrical charge due to the loss or gain of one or more electrons.
Molecule: A group of two or more atoms bonded together, representing the smallest fundamental unit of a chemical compound.
Compound: A substance formed when two or more different elements are chemically bonded together.
Empirical Formula: The simplest whole-number ratio of atoms of each element in a compound.
Molecular Formula: The actual number of atoms of each element in a compound.

Mass Definitions

The relative mass of an atom is based on the carbon-12 isotope:

One atom of carbon-12 has a mass of 1.992646538 \times 10^{-26} kg.
This mass can be simplified to 12 atomic mass units (u).

Here are some key concepts related to mass calculations:

Relative Isotopic Mass: The mass of a specific isotope compared to 1/12 of a carbon-12 atom.
Relative Atomic Mass (A): The weighted average mass of an atom relative to 1/12 the mass of a carbon-12 atom, accounting for isotopic composition.
Relative Formula Mass (M): The total mass of a compound calculated from the sum of relative atomic masses of its constituent atoms.

Examiner Tips and Tricks

When calculating relative atomic mass values, use the periodic table for accuracy, as exemplified in calculating the mass of potassium carbonate.

Avogadro & the Mole

Mole Concept

Avogadro's constant (denoted as N or L) is the number of particles (atoms, molecules, etc.) in one mole, about 6.02 \times 10^{23} particles/mole. Here’s how it connects to calculations:

1 mole of any element equals its relative atomic mass in grams.
Example: 1 mole of carbon = 12.00 g.

Worked Examples

Determining Molar Mass: For water (H₂O):
Molar mass = (2×1) + 16 = 18 g/mol.
Calculating Moles: For 100 g of water:
n = m/M = 100/18 = 5.56 moles.
Molecules in Water: Number of molecules = moles × Avogadro's constant = 5.56 × 6.02 \times 10^{23} = 3.35 \times 10^{24} molecules.

Full & Ionic Equations

Balancing Equations

Equations must be balanced to show matter conservation:

Atom count on reactants must equal products.
Balance hydrogen and oxygen last in combustion reactions.

State Symbols

Indicate physical states in reactions:

(s) - solid
(l) - liquid
(g) - gas
(aq) - aqueous solution

Writing Ionic Equations

To derive ionic equations:

Identify dissociated ions in the aqueous state.
Cancel spectator ions, showing only reacting participants.

Types of Reactions

Illustrate types of reactions with examples:

Displacement: More reactive element displaces a less reactive in a compound.
Neutralisation: Acid reacts with a base to form salt and water.
Precipitation: Formation of a solid from two solutions.

Calculating Formulae

Empirical vs. Molecular Formulas

.empirical formula conveys the simplest ratio; molecular formula details the actual count of atoms.

Empirical formula can derive from mass or percentages in a compound.
Molecular formula is found by multiplying empirical formula by a whole number to match molecular mass.

Worked Examples

From Mass: Obtain empirical formula from specific quantities of elements (e.g., 10 g of H, 80 g of O).
From Percentage: Example: 85.7% C and 14.3% H gives empirical formula C_xH_y.

Percentage Composition

Calculate percentage by mass of an element in a compound using:

%\text{ mass of an element}= \frac{A_{r} \times \text{ number of atoms of the element}}{M_{r}\text{ of the compound}}\times 100

Water of Crystallisation

Some compounds can incorporate water into their crystalline structure:

Hydrated Compounds: Exhibit water in their structure (e.g., CuSO₄·5H₂O).
Anhydrous Compounds: Lack water (e.g., CuSO₄).

Ideal Gas Equation

Used for gas calculations:

PV=nRT

P = pressure (Pa)
V = volume (m³)
n = moles
R = gas constant (8.31 J mol^-1 K^-1)
T = temperature (K)

Worked Example with Hydrated Copper Sulfate

Calculate properties upon heating to find water of crystallisation.

Conclusion

The understanding of formulae, equations, moles, and molar relationships in Chemistry is crucial for mastering chemical principles and calculations. Continuous practice and application of these concepts in various scenarios will enhance proficiency in Chemistry.

Comprehensive Guide to Formulae & Equations in IAL Chemistry

Comprehensive Guide to Formulae & Equations in IAL Chemistry