9701 · 2.2
The mole and the Avogadro constant flashcards
Revision flashcards for Cambridge 9701 The mole and the Avogadro constant (syllabus 2.2). Flip, recall, then mark a real past-paper question.
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What is the definition of a mole?
The amount of substance that contains the same number of specified elementary entities as there are atoms in exactly 12 g of the carbon-12 isotope. This number is the Avogadro constant.
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What is the value and symbol for the Avogadro constant?
The Avogadro constant ($L$ or $N_A$) is $6.02 \times 10^{23}$ mol^{-1}.
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What is molar mass ($M_r$)?
The mass of one mole of a substance, expressed in grams per mole (g mol^{-1}). Its numerical value is the same as the relative molecular/formula mass.
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What is the formula linking moles ($n$), mass ($m$), and molar mass ($M_r$)?
$n = \frac{m}{M_r}$
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What is the formula linking concentration ($c$), moles ($n$), and volume ($V$)?
$c = \frac{n}{V}$
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What are the standard units for concentration in A-Level Chemistry?
moles per decimetre cubed (mol dm^{-3})
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How do you convert a volume from \mathrm{cm}^{3} to \mathrm{dm}^{3}?
Divide by 1000. For example, 250 \mathrm{cm}^{3} is equal to 0.250 \mathrm{dm}^{3}.
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What is the molar volume of an ideal gas at room temperature and pressure (r.t.p.)?
$24\ \mathrm{dm}^{3}\ \mathrm{mol}^{-1}$ (or $24{,}000\ \mathrm{cm}^{3}\ \mathrm{mol}^{-1}$). This means one mole of any ideal gas occupies $24\ \mathrm{dm}^{3}$ at r.t.p.
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What does r.t.p. stand for?
Room Temperature and Pressure. This is defined as approximately 298 K (25 °C) and 1 atm (101 kPa) pressure.
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What is a common mistake when calculating the number of atoms in a compound?
Forgetting to multiply the moles of the compound by the number of atoms of that element within one formula unit. E.g., 1 mole of H₂O contains 2 moles of H atoms.
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Why is it important to use the correct significant figures in mole calculations?
Exam answers must be given to an appropriate number of significant figures, usually determined by the least precise data value given in the question (typically 2 or 3 s.f.).