Moles and Mass

• What is the definition of a mole in chemistry?


  A mole is simply the name given to an amount of a substance.
• How does the mole relate to large numbers?


  The mole represents a specific number of particles, which is 6.02 × 10²³.
• What is the Avogadro constant?


  The Avogadro constant is 6.02 × 10²³ particles.
• What does one mole of any substance contain?


  One mole of any substance contains 6.02 × 10²³ particles.
• What is the relationship between the mass of a mole of a substance and its relative atomic or molecular mass?


  The mass of one mole of a substance is equal to its relative atomic mass (A_r) or relative formula mass (M_r) in grams.
• How much does one mole of carbon weigh?


  One mole of carbon weighs exactly 12 g.
• What is the mass of one mole of nitrogen gas (N₂)?


  One mole of nitrogen gas (N₂) weighs exactly 28 g.
• How much does one mole of carbon dioxide (CO₂) weigh?


  One mole of carbon dioxide (CO₂) weighs exactly 44 g.
• What does it mean if 12 g of carbon, 28 g of N₂, and 44 g of CO₂ all contain the same number of particles?


  It means that they all contain one mole of particles, which is 6.02 × 10²³.
• How do you calculate the number of moles in a given mass?


  • Number of moles = mass in g (of an element or compound) ÷ M_r (of the element or compound)
• How many moles are there in 66 g of carbon dioxide (CO₂)?


  There are 1.5 moles in 66 g of carbon dioxide.
• How do you calculate the M_r of carbon dioxide (CO₂)?


  The M_r of CO₂ is calculated as 12 + (16 × 2) = 44.
• What mass of carbon is there in 4 moles of carbon dioxide?


  The mass of carbon in 4 moles of carbon dioxide is 48 g.
• What does the conservation of mass state during a chemical reaction?


  Mass is always conserved; the same number and types of atoms are present on each side of the reaction equation.
• How can you show that mass is conserved in the reaction 2Li + F₂ → 2LiF?


  By counting the number of lithium and fluorine atoms on each side of the equation, which are equal.
• What happens if the mass seems to change during a reaction?


  It could be due to a gas being involved that is escaping or entering the reaction vessel.
• What happens if the mass increases during a reaction?


  It likely means a gas from the air has reacted and is now contained in the reaction vessel.
• What happens if the mass decreases during a reaction?


  It likely means a gas product has escaped from the reaction vessel.
• How can you show that mass is conserved in the reaction H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O?


  By ensuring the total mass of reactants equals the total mass of products.
• How can moles be used in chemical reactions?


  • Moles can calculate masses in reactions.
  • Moles can balance equations using reacting masses.
• What is the balanced equation for the reaction of zinc oxide (ZnO) with carbon?


  The balanced equation is ZnO + C → CO₂ + Zn.
• What is relative atomic mass represented by?


  Ar
• What does the notation $^{35}_{17}Cl$ indicate?


  It indicates the number of protons and neutrons in the chlorine isotope
• How is relative atomic mass calculated?


  It is the average mass of all isotopes of an element
• What is relative formula mass denoted as?


  Mr
• How is relative formula mass calculated?


  By adding together the relative atomic masses of all atoms in a compound's molecular formula
• What is the relative formula mass of MgCl$_2$?


  $95$
• How do you calculate the percentage mass of a particular element in a compound?


  By using the formula: $\frac{Ar \text{ of element} \times \text{no. of element} \times 100}{Mr}$
• What is the value of Avogadro's constant?


  $6.02 \times 10^{23}$
• What does one mole of any substance represent?


  It represents $6.02 \times 10^{23}$ particles of that substance
• How do you calculate the number of moles from mass?


  Using the formula: $\text{Number of moles} =$$\frac{\text{mass}}{Mr}$
• What are the balanced chemical equations for the reactions involving magnesium and hydrochloric acid?

  1. $1Mg +$$2 HCl \rightarrow 1MgCl_2 +$$1H_2$
  2. $2Mg +$$4HCl \rightarrow 2MgCl_2 +$$2H_2$
• What is the first step in calculating mass in reactions?


  Write balanced equations
• Why do we find moles of the known substance in a reaction?


  To use it as a basis for finding moles of the unknown substance
• If you have 5g of sodium, how do you find the moles?


  Use the formula moles = mass/Mr
• What is the molar mass (Mr) of sodium?


  23 g/mol
• How many moles of sodium are in 5g?


  $0.217$
• What is the molar ratio in the reaction $4Na +$$O₂ → 2Na₂O$?


  1. 2 or 2:1
• How do you find the moles of Na₂O from the moles of sodium?


  Divide the moles of sodium by 2
• What is the molar mass (Mr) of Na₂O?


  62 g/mol