Relative Masses of Atoms and MoleculesCambridge IGCSE Chemistry: Flashcards
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Define relative atomic mass (Ar).
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- Define relative atomic mass (Ar).
- The average mass of the isotopes of an element, compared to 1/12th of the mass of an atom of carbon-12.
- Define relative molecular mass (Mr).
- The sum of the relative atomic masses of all the atoms in a molecule.
- What is relative formula mass used for?
- The same calculation as relative molecular mass, but applied to ionic compounds, which do not exist as individual molecules.
- Do Ar and Mr have units? Why or why not?
- No — they are ratios (comparisons to carbon-12), not actual masses, so they have no units.
- What does the molecular formula of a compound show?
- The number and type of different atoms present in one molecule of the compound.
- Calculate the Mr of water, H2O (Ar: H=1, O=16).
- Mr = (2 x 1) + 16 = 18.
- Calculate the Mr of calcium carbonate, CaCO3 (Ar: Ca=40, C=12, O=16).
- Mr = 40 + 12 + (3 x 16) = 100.
- What common mistake do students make when calculating Mr from a formula with subscripts?
- Forgetting to multiply an atom's mass by its subscript number, e.g. treating H2O as H + O instead of (2 x H) + O.
- What is molar mass, and how does it relate to Ar/Mr?
- The mass of one mole of a substance, in g/mol; it has the same numerical value as the substance's Ar or Mr.
- State the equation linking amount of substance, mass and molar mass.
- amount of substance (mol) = mass (g) ÷ molar mass (g/mol).
- How many moles are in 36 g of water (Mr = 18)?
- 36 ÷ 18 = 2 moles.