The Mole & the Avogadro Constant (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Flashcards
Exam code: 0654 & 0973
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What is the Avogadro constant?
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What is the Avogadro constant?
The Avogadro constant is 6.02 × 1023. It is the number of particles (atoms, molecules or ions) in one mole of any substance.
True or False?
One mole of any gas occupies a volume of 24 dm3 at room temperature and pressure (RTP).
True.
At RTP (20°C and 1 atm), one mole of any gas occupies 24 dm3 (24,000 cm3). This is the molar gas volume at RTP.
One mole of a compound has a mass (in grams) equal to its ______ mass. One mole of CO2 (Mr = ______) has a mass of ______ g.
One mole of a compound has a mass (in grams) equal to its relative molecular mass. One mole of CO2 (Mr = 44) has a mass of 44 g.
How many molecules are present in 2 moles of water?
2 moles × 6.02 × 1023 molecules per mole = 1.20 × 1024 molecules of water.
True or False?
One mole of sodium and one mole of sodium chloride both contain 6.02 × 1023 particles.
True.
One mole of any substance always contains 6.02 × 1023 of the stated particles, whether those are atoms, molecules or formula units.
The volume (dm3) of a gas at RTP = moles × ______. So, 0.5 moles of oxygen occupies ______ dm3 at RTP.
The volume (dm3) of a gas at RTP = moles × 24. So, 0.5 moles of oxygen occupies 12 dm3 at RTP.
Why do equal volumes of different gases contain the same number of molecules at the same temperature and pressure?
At the same temperature and pressure, the average spacing between gas molecules is the same regardless of the type of gas. Equal volumes therefore always contain equal numbers of molecules (Avogadro's Law), regardless of the molecular mass.
State the equation linking moles, mass and Mr.
Moles = mass (g) ÷ Mr.
This rearranges to: mass = moles × Mr, or Mr = mass ÷ moles.
True or False?
One mole of magnesium has a mass of 24 g because the Ar of magnesium is 24.
True.
The mass of one mole of an element in grams equals its relative atomic mass. Since Ar(Mg) = 24, one mole of magnesium has a mass of 24 g.
To calculate moles, divide ______ (in grams) by the ______. To calculate mass, multiply ______ by the Mr.
To calculate moles, divide mass (in grams) by the Mr. To calculate mass, multiply moles by the Mr.
What mass of zinc is contained in 0.250 moles of zinc? (Ar: Zn = 65)
Mass = moles × Mr = 0.250 × 65 = 16.25 g.
True or False?
44 g of CO2 (Mr = 44) contains exactly 1 mole of CO2.
True.
Moles = mass ÷ Mr = 44 ÷ 44 = 1 mole. One mole of a substance always has a mass equal to its Mr in grams.
9 g of water (Mr = 18) contains ______ moles of water molecules, which is ______ × 1023 molecules.
9 g of water (Mr = 18) contains 0.5 moles of water molecules, which is 3.01 × 1023 molecules.
How many moles are in 2.64 g of sucrose (Mr = 342)?
Moles = mass ÷ Mr = 2.64 ÷ 342 = 7.72 × 10-3 mol (0.00772 mol).
What is the limiting reactant in a chemical reaction?
The limiting reactant is the reactant that is completely used up first. It limits the amount of product that can form. The other reactant remains in excess.
True or False?
In a reacting masses calculation, the molar ratio from the balanced equation is used to convert moles of reactant to moles of product.
True.
The coefficients in a balanced equation give the molar ratio of reactants to products. This ratio is applied after converting mass to moles.
In a reacting masses calculation: convert ______ to moles (mass ÷ Mr), apply the ______ ratio from the balanced equation, then convert moles back to ______.
In a reacting masses calculation: convert mass to moles (mass ÷ Mr), apply the molar ratio from the balanced equation, then convert moles back to mass.
In the reaction 2Mg + O2 → 2MgO, what mass of MgO is produced from 6.0 g of Mg? (Ar: Mg = 24, O = 16)
Moles of Mg = 6.0 ÷ 24 = 0.25 mol. Ratio Mg : MgO = 1 : 1, so 0.25 mol MgO produced. Mass of MgO = 0.25 × 40 = 10 g.
True or False?
The total mass of products in a chemical reaction is always less than the total mass of reactants.
False.
By the law of conservation of mass, the total mass of products always equals the total mass of reactants. Atoms are rearranged but not created or destroyed.
How do you identify the limiting reactant when given masses of two reactants?
Convert both masses to moles. Compare with the molar ratio from the balanced equation to determine which reactant would run out first. The one that runs out is the limiting reactant; the other is in excess.
The reactant that is ______ up first is the ______ reactant. The amount of product is determined by the amount of the ______ reactant.
The reactant that is used up first is the limiting reactant. The amount of product is determined by the amount of the limiting reactant.
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