9701 · 2.3
Formulas flashcards
Revision flashcards for Cambridge 9701 Formulas (syllabus 2.3). Flip, recall, then mark a real past-paper question.
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Define 'Empirical Formula'.
The simplest whole-number ratio of atoms of each element present in a compound.
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Define 'Molecular Formula'.
The actual number of atoms of each element in a single molecule of a compound.
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What is the mathematical relationship between molecular and empirical formulas?
Molecular Formula = (Empirical Formula)$_n$, where 'n' is a positive integer representing the ratio between the molecular and empirical formula masses.
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How do you calculate the integer 'n' to find the molecular formula?
n = (Relative Molecular Mass, $M_r$) / (Relative Empirical Formula Mass)
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When are the empirical and molecular formulas the same?
When the integer 'n' is 1. This occurs when the simplest ratio is also the actual number of atoms, e.g., for water (H₂O) or methane (CH₄).
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What is a 'formula unit'?
The simplest whole-number ratio of ions in an ionic compound. For ionic compounds, the empirical formula is referred to as the formula unit.
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Why don't we use the term 'molecular formula' for sodium chloride (NaCl)?
Because NaCl is an ionic compound that forms a giant ionic lattice, not discrete molecules. The formula 'NaCl' represents the 1:1 ratio of ions in this lattice.
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What is the first step when calculating an empirical formula from percentage composition?
Assume a 100 g sample of the compound. This allows you to convert the percentages directly into masses in grams for each element.
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What is the empirical formula of glucose, C₆H₁₂O₆?
CH₂O. The subscripts (6, 12, 6) are all divided by their highest common factor, which is 6.
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A compound has an empirical formula of CH and a molar mass of 78.0 g mol⁻¹. What is its molecular formula?
Empirical formula mass of CH = 12.0 + 1.0 = 13.0. n = 78.0 / 13.0 = 6. The molecular formula is (CH)₆ = C₆H₆ (benzene).
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In an empirical formula calculation, the mole ratio is 1 : 1.33. How do you convert this to a whole-number ratio?
Recognise that 1.33 is approximately 4/3. Multiply the entire ratio by 3 to get the whole-number ratio 3 : 4.