RELATIVE FORMULA or MOLECULAR MASS

Doc Brown's Chemistry - GCSE/IGCSE/GCE (basic A level) O Level Online Chemical Calculations

Quantitative Chemistry calculations online Help for problem solving in doing relative formula mass calculations using atomic masses. Practice revision questions on calculating relative molecular mass from a chemical formula and atomic masses. This page describes, and explains, with worked out examples, the method of how to calculate the relative formula mass of a compound (ionic or covalent) or the relative molecular mass of an element or a covalent compound. Online practice exam chemistry CALCULATIONS and solved problems for KS4 Science GCSE/IGCSE CHEMISTRY and basic starter chemical calculations for A level AS/A2/IB courses. These revision notes and practice questions on calculating the relative formula mass or calculating relative molecular mass calculations in chemistry and worked examples should prove useful for the new AQA, Edexcel and OCR GCSE (9–1) chemistry science courses.

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 Self-assessment Quizzes on relative formula mass or relative molecular mass:

The relative molecular mass/relative formula mass is defined as the sum of all the individual atomic masses of ALL the atoms in the formula (M_r).

If the individual atomic masses of all the atoms in a formula are added together you have calculated the relative formula mass

Atomic masses are listed at the bottom of the page

e.g. for ionic compounds e.g. NaCl = 23 + 35.5 58.5) or molecular mass for covalent elements or compounds ...

e.g. M_r of N₂ = 28 from (2 x 14) or compounds e.g. M_r of C₆H₁₂O₆ = 180 from [(6 x 12) + (12 x 1) + (6 x 16)],

and more examples of how to calculate relative formula mass are further down the page, you get atomic masses from your periodic table.

In a balanced chemical symbol equation, the total of relative formula masses of the reactants is equal to the total relative formula masses of the products (see law of conservation of mass calculations).

To be honest, the term relative formula mass can be used with any compound whether it be ionic or covalent - it just seems NOT correct to talk about the molecular mass of an ionic compound when it doesn't consist of molecules, but is that one for the purists!

The shorthand M_r can be used for the formula of any element or compound and to repeat, 'it doesn't matter whether a compound is ionic or covalent'.

M_r = Relative formula mass = relative molecular mass

= the sum of all the atomic masses for ALL the atoms in a given formula

NOTE: You cannot successfully calculate formula/molecular masses if you cannot read a formula correctly!

Whereas relative atomic mass (section 1. Relative Atomic Mass) only applies to a single atom, anything with at least two atoms in the formula requires the term relative formula mass or relative molecular mass to be used.

WARNING: The most common error is to use atomic/proton numbers instead of atomic masses, unfortunately, except for hydrogen, they are different!

Examples of relative formula mass or relative molecular mass calculations:

How to calculate relative molecular mass = How to calculate relative formula mass

Molecular/formula mass = total of all the atomic masses of ALL the atoms in the molecule/compound.

Watch out for brackets e.g. (OH)₂ means two OH groups to add up!

• Relative molecular mass/formula mass calculation Example 2.1
  • The diatomic molecules of the elements hydrogen H₂ and chlorine Cl₂
  • relative atomic masses, Ar: H = 1, Cl = 35.5
  • Formula masses, RMM or M_r
    • relative molecular mass for hydrogen H₂ = 2 x 1 = 2
    • relative molecular mass for chlorine Cl₂ = 2 x 35.5 = 71 respectively.
• Relative molecular mass/formula mass calculation Example 2.2
  • The element phosphorus consists of P₄ molecules. (atomic mass of P = 31)
  • relative molecular mass or M_r of phosphorus = 4 x its atomic mass = 4 x 31 = 124
• Relative molecular mass/formula mass calculation Example 2.3: The compound water H₂O
  • relative atomic masses are H=1 and O=16
  • relative molecular mass or M_r = (1x2) + 16 = 18 (molecular mass of water)
• Relative molecular mass/ relative formula mass calculation Example 2.4
  • The compound sulphuric acid H₂SO₄  when pure, is a covalent compound
  • relative atomic masses are H = 1, S = 32 and O = 16
  • relative molecular mass or M_r = (1 x 2) + 32 + (4 x 16) = 98 (molecular mass of sulphuric acid )
• Relative formula mass calculation Example 2.5
  • The compound magnesium hydroxide Mg(OH)₂ (ionic)
  • relative atomic masses are Mg = 24, H = 1 and O = 16
  • relative formula mass or M_r = 24 + 2 x (16+1) = 58
  • Important note on terminology
    • The term relative formula mass is best applied to giant structure compounds
      • e.g. ionic compounds like magnesium hydroxide (above),
      • and silicon dioxide SiO₂ (RFM = 28 + 16 + 16 = 60), a 3D giant covalent lattice compound.
• Relative formula mass calculation Example 2.6
  • The ionic compound aluminium oxide (Al³⁺)₂(O^2-)₃ or just the plain formula Al₂O₃
  • but it makes no difference to the calculation of relative formula mass or relative molecular mass.
  • relative atomic masses are Al = 27 and O = 16
  • so the relative formula mass RFM or M_r = (2 x 27) + (3 x 16) = 102
• Relative formula mass calculation Example 2.7
  • Calcium phosphate is also ionic but a more tricky formula to work out!
  • (Ca²⁺)₃(PO₄^3-)₂ or Ca₃(PO₄)₂, but it makes no difference to the calculation of relative formula mass or relative molecular mass.
  • atomic masses: Ca = 40, P = 31, O =16
  • relative formula mass or M_r = (3 x 40) + 2 x {31 + (4 x 16)} = (120) + (2 x 95) = 310
• Relative molecular mass/formula mass calculation Example 2.8
  • Glucose C₆H₁₂O₆  a covalent compound
  • atomic masses: C = 12, O= 16, H = 1
  • relative molecular mass of glucose M_r(C₆H₁₂O₆) = (6 x 12) + (12 x 1) + (6 x 16) = 180
• Relative molecular mass calculation Example 2.9
  • butane C₄H₁₀
  • relative atomic masses: C = 12, H=1
  • M_r = (4 x 12) + (10 x 1) = 58
• Relative formula mass calculation Example 2.10
  • copper(II) sulfate (copper sulfate, ionic)
  • relative atomic masses: Cu = 63.5, S = 32, O = 16
  • M_r = 63.5 + 32 + (4 x 16) = 159.5 (its 160 is you use Cu = 64)
• Relative molecular mass calculation Example 2.11
  • propanol, C₃H₈O, CH₃CH₂CH₂OH (the same formula can be expressed in different ways!)
  • relative atomic masses: C = 12, H=1, O = 16
  • M_r = (3 x 12) + (8 x 1) + 16 = 60
• Relative formula mass calculation Example 2.12
  • magnesium nitrate, Mg(NO₃)₂
  • relative atomic masses: Mg = 24, N = 14, O = 16
  • M_r = 24 + (2 x 14) + (6 x 16) = 24 + 28 + 96 = 148
• Relative formula mass calculation Example 2.13
  • blue hydrated copper sulfate crystals CuSO₄.5H₂O, again a bit more tricky!
  • Cu = 63.5, S = 32, H = 1, O =16
  • Just do this carefully in parts eg
  • CuSO₄ = 63.5 + 32 + (4 x 16) = 159.5 (160 if you use Cu = 64)
  • H₂O = (2 x 1) + 16 = 18, 5 x 18 = 90
  • Formula mass = 159.5 + 90 = 249.5 (250 if Cu = 64)

Self-assessment Quizzes on relative formula mass or relative molecular mass

OTHER CALCULATION PAGES

1. What is relative atomic mass?, relative isotopic mass and calculating relative atomic mass

2. Calculating relative formula/molecular mass of a compound or element molecule (this page)

3. Law of Conservation of Mass and simple reacting mass calculations

4. Composition by percentage mass of elements in a compound

5. Empirical formula and formula mass of a compound from reacting masses (easy start, not using moles)

6. Reacting mass ratio calculations of reactants and products from equations (NOT using moles) and brief mention of actual percent % yield and theoretical yield, atom economy and formula mass determination

   • Reacting masses, concentration of solution and volumetric titration calculations (NOT using moles)

7. Introducing moles: The connection between moles, mass and formula mass - the basis of reacting mole ratio calculations (relating reacting masses and formula mass)

8. Using moles to calculate empirical formula and deduce molecular formula of a compound/molecule (starting with reacting masses or % composition)

9. Moles and the molar volume of a gas, Avogadro's Law

10. Reacting gas volume ratios, Avogadro's Law and Gay-Lussac's Law (ratio of gaseous reactants-products)

11. Molarity, volumes and solution concentrations (and diagrams of apparatus)

12. How to do acid-alkali titration calculations, diagrams of apparatus, details of procedures

13. Electrolysis products calculations (negative cathode and positive anode products)

14. Other calculations e.g. % purity, % percentage & theoretical yield, dilution of solutions (and diagrams of apparatus), water of crystallisation, quantity of reactants required, atom economy

    • 14.1 % purity of a product 14.2a % reaction yield 14.2b atom economy 14.3 dilution of solutions

    • 14.4 water of crystallisation calculation  14.5 how much of a reactant is needed? limiting reactant

15. Energy transfers in physical/chemical changes, exothermic/endothermic reactions

16. Gas calculations involving PVT relationships, Boyle's and Charles Laws

17. Radioactivity & half-life calculations including dating materials

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