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5.1.2Additional lessons

Reacting Mass Calculations — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 90 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5The Core Relationships

Every reacting-mass problem passes through the amount in moles.

Moles from mass
  • Moles from mass n = m ÷ M, where m is the mass in grams and M is the molar mass in g mol⁻¹.
Molar mass
  • Molar mass The sum of the relative atomic masses of all atoms in the formula, expressed in grams per mole.
Mole ratio
  • Mole ratio The coefficients in the balanced equation give the ratio in which moles of substances react and form.
Mass from moles
  • Mass from moles m = n × M converts the calculated moles of the required substance into a mass in grams.

Section 2 of 5Four-Step Strategy

1
Write a balanced equation for the reaction, including all relevant species.
2
Convert the given mass to moles using n = m ÷ M, with M taken from the periodic table.
3
Use the mole ratio from the equation to find the moles of the substance you want.
4
Convert those moles to mass using m = n × M and check the number of significant figures.
5
Compare the answer with the starting mass to see whether it is chemically sensible.

Section 3 of 5Good Practice

Good Practice
Significant figures
  • Significant figures Give the answer to the same number of significant figures as the least precise data value.
Rounding
  • Rounding Keep extra digits during the calculation and round only at the end to avoid accumulated errors.
Units
  • Units Write units on every line, and check that grams cancel to leave moles.
Sense check
  • Sense check The mass of products in a closed reaction must equal the mass of reactants used.

Section 4 of 5Worked Example

Question

  • Calculate the mass of magnesium chloride formed when 2.43 g of magnesium reacts with excess hydrochloric acid: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
  • (Ar: Mg = 24.31, Cl = 35.45)

  1. n(Mg) = 2.43 ÷ 24.31 = 0.09996 mol.
  2. Mole ratio MgMgCl₂ = 1 : 1, so n(MgCl₂) = 0.09996 mol.
  3. M(MgCl₂) = 24.31 + 2 × 35.45 = 95.21 g mol⁻¹.
  4. m = 0.09996 × 95.21 = 9.52 g.

Answer 9.52 g of MgCl₂

Section 5 of 5Exam Checklist & Tips

5.1.2 – Key Points for the Exam

  • Mole The amount of substance containing 6.02 × 10²³ elementary entities.
  • Molar mass Mass of one mole of a substance in g mol⁻¹.
  • Stoichiometry The quantitative relationship between reactants and products in a reaction.
  • Mole ratio The ratio of coefficients used to compare amounts of two substances.

Common Mistakes

  • Using the mass ratio from the equation instead of the mole ratio.
  • Forgetting to multiply the relative atomic mass of an element by its subscript.
  • Rounding intermediate values too early, affecting the final answer.
  • Using the mass of the wrong substance as the starting point of the calculation.

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