C3- Quantitative Chemistry

• What does the law of conservation of mass state?

  No atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.
• What does it mean for a chemical equation to be balanced?

  It means there are exactly the same number of atoms of each element on both sides of the equation.
• How is the relative formula mass (Mr) of a compound calculated?

  It is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
• What is the relative formula mass of HCl?

  $Mr =$$1 +$$35.5 =$$36.5$
• What does a balanced chemical equation imply about the mass of reactants and products?

  The sum of the Mr of reactants in quantities shown equals the sum of the Mr of products in quantities shown.
• What should you check if a reaction appears to involve a change in mass?

  You should check if this is due to a reactant or product being a gas and its mass not being accounted for.
• What happens when a metal reacts with oxygen in terms of mass?

  The mass of the metal oxide product is greater than the mass of the metal.
• What is always present in a measurement made in chemistry?

  There is always some uncertainty about the result obtained.
• How should you prepare when making measurements in chemistry?

  Be prepared to make estimations of uncertainty.
• How are chemical amounts measured?

  Chemical amounts are measured in moles, with the symbol mol.
• What is the mass of one mole of a substance in grams equal to?

  It is numerically equal to its relative formula mass.
• If the Mr of iron is 56, what is the mass of one mole of iron?

  56 g
• How many grams are in one mole of nitrogen gas, N₂?

  28 g
• What does one mole of a substance contain?

  It contains the same number of stated particles, atoms, molecules, or ions as one mole of any other substance.
• What is the Avogadro constant?

  1. 02 x 10²³ per mole.
• How can masses of reactants and products be calculated?

  They can be calculated from balanced symbol equations.
• How can chemical equations be interpreted?

  They can be interpreted in terms of moles.
• In the equation Mg + 2HCl -> MgCl₂ + H₂, how many moles of Mg react with 2 moles of HCl?


  1 mol of Mg reacts with 2 mol of HCl.
• What must be the same on both sides of a balanced equation?

  The total moles of one element must be the same on both sides of the equation.
• How can balancing numbers in a symbol equation be calculated?

  By converting the masses in grams to amounts in moles and then to simple whole number ratios.
• What is a limiting reactant?

  The limiting reactant is the reactant that is used up and not in excess, limiting the amount of products formed.
• What happens to the amount of product produced if a limiting reactant is used?

  The amount of product produced is restricted to the amount of the excess reactant that reacts with the limiting one.
• How can concentration of a solution be measured?

  Concentration can be measured in mass per given volume of solution, e.g., grams per dm³ (g/dm³).
• How do you calculate the mass of solute in a given volume of a known concentration?

  Use the formula mass = concentration x volume, i.e., g = g/dm³ x dm³.
• What effect does a smaller volume or larger mass of solute have on concentration?

  It gives a higher concentration.
• What effect does a larger volume or smaller mass of solute have on concentration?

  It gives a lower concentration.