Counting particles by mass: The mole - IB DP Chemistry- Study Notes - New Syllabus 2025
Counting particles by mass: The mole – IB DP Chemistry- Study Notes
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S1.4.1 – Mole
Mole:
- A fixed number of particles, refers to the amount, n, of substance
- A chemical species may be an atom, a molecule, or an ion
- The same as using the terminology “particles” for atoms, molecules, and formula units
Definition of a Mole:
- The amount of a substance that contains the same number of chemical species as there are atoms in exactly 12g of the isotope carbon-12.
Avogadro’s constant (NA) has the value 6.02 x 1023 particles per mole
Moles and Number particles relationship:
- N = number of particles
- NA= Avogadro’s Constant
- the conversion factor linking the number of particles and amount of substance in moles. It has the unit of mol-1
- n = number of moles N = n × NA
Example:
- Calculate the amount of lead (Pb), in mol and mmol, in a sample containing 2.9 × 10 21atoms of this element.
- mmol = 10-3mol
- 8×10-3 x 10-3= 4.8 mmol
S1.4.2-3 – Relative Atomic Mass and Molar Mass
Relative Atomic Mass and Molecular Mass:
Relative Atomic Mass (Ar):
- The ratio of the mass of a certain atom to one-twelfth of the mass of a carbon-12 atom.
Relative Molecular Mass (Mr):
- Ratio of the mass of a molecule or other multiatomic species to one-twelfth of the mass of a carbon-12 atom.
- Both Arand Mr are ratios with no units
Hydrates:
- Many ionic compounds form hydrates:
- Compounds in which water molecules form coordination bonds with the ions
Molar Mass (M):
- The molar mass of a substance is 1 mol of that substance.
- Molar mass is numerically equal to the relative atomic mass
Example from Book (page 83 (Q41) and 91 (Q44)):
S1.4.4 – Empirical formula and Molecular Formula and Chemical Analysis (in progress)
Percentage Composition:
- The elemental composition of a compound is often expressed in percent by mass, which is commonly referred to as the percentage composition.
- The mole ratio can be used to calculate the percentage composition of a compound
Empirical vs. Molecular Formula:
- Empirical Formula
- Simplest whole number ratio of the atoms it contains
2.Molecular Formula
- Multiple of the empirical formulas showing the actual number of atoms of each element present.
Determining the Empirical Formula:
A compound of iron and oxygen is analyzed and found to contain 69.94% iron and 30.06% oxygen. Find the empirical formula of the compound.
S1.4.5: Solutions and Concentrations
Solution:
- Homogeneous mixtures of two or more components. Each solution consists of a solventand one or more
- The solvent is usually the major component of the solution, so the properties of the whole solution are like the properties of the solvent. The other components of the solution are called solutes
- An aqueous Solution is a solution in which water is a solvent
Concentrated and Dilute:
- A concentrated solution contains a large proportion of solute, and so has a high ratio of solute to solvent.
- A dilute solution has a small proportion of solute, and so has a low ratio of solute to solvent.
Mass Concentration:
- The composition of a solution is sometimes expressed as the mass concentration, ρ solute, of the solute.
- Grams of solute dissolved per $dm^{3}$ of the solution.
- The most common units for mass concentration are $gdm^{–3}$ and $gcm^{–3}$ .
Molar Concentration (Molarity):
- Moles of solute per dm3of the solution
- Common unit: dm3
- For smaller:
- 1 mmol $dm^{-3} = 1 x 10^{-3} mol dm^{-3}$
- 1 µmol $dm^{-3} = 1 x 10^{-6} mol dm^{-3}$
Dilution:
- Practice done to store chemicals in the form of a concentrated solution
- They dilute them to the required concentration when needed.
- Solutions with a known concentration of the solute are called Standard Solutions
- Formula:
- C1V1= C2V2
S1.4.6 – Avogadro’s Law