quantitative chemistry

• How is the relative atomic mass of an element defined?

  It is the relative mass of its atoms compared to the mass of a carbon-12 atom.
• Where can the A_r values for elements be found?

  In the periodic table.
• What does the relative atomic mass have in terms of units?

  It has no units.
• What is the procedure for calculating relative formula mass (M_r)?


  1. Determine the number of atoms of each element in the chemical formula.
  2. Add together the A_r values for all the atoms of each element present.
• What is the relative formula mass of carbon dioxide (CO₂)?

  44
• How can the relative formula mass of CO₂ be calculated?

  By adding the relative atomic mass of 1 carbon atom (12) and 2 oxygen atoms (16 each): $12 +$$(2 \times 16) =$$44$.
• Do ionic compounds exist as molecules?

  No, ionic compounds do not exist as molecules.
• How are the relative formula masses of ionic compounds calculated?

  In the same way as molecular compounds, from the numbers shown in the formula.
• What does the law of conservation of mass state?

  No atoms are lost or made in a chemical reaction.
• Why is the number of atoms of each element the same on both sides of a balanced symbol equation?

  Because atoms are neither lost nor made in a chemical reaction.
• What happens to the total mass of products and reactants in a chemical reaction?

  The total mass of the products is equal to the total mass of the reactants.
• What happens to the total mass in a closed system during a reaction?

  The total mass remains constant.
• What is a closed system in the context of chemical reactions?

  A closed system is one where no substances can enter or leave.
• What occurs in a non-enclosed system during a reaction?

  Substances can enter or leave the system.
• What happens to the total mass if a gas escapes from a non-enclosed system?

  The total mass will appear to decrease.
• How is uncertainty in chemical measurements defined?

  There is always some uncertainty in the result obtained.
• What are two ways of estimating uncertainty in measurements?

  By considering the resolution of measuring instruments and from the range of a set of repeat measurements.
• What is the resolution of a measuring instrument?

  It is the smallest change in a quantity that gives a change in the reading that can be seen.
• How is uncertainty estimated from a set of repeat measurements?

  The uncertainty is ± half the range.
• What is concentration in the context of solutions?

  • A measure of how 'crowded' the solute particles are.
  • More concentrated solutions contain more particles in a given volume.
• How is the concentration of a solution calculated?

  Using the mass of dissolved solute in grams and the volume of solution in cubic decimetres.
• What are the units for concentration?

  g/dm³.
• How can the units for concentration also be shown?

  As g dm³, which means the same as g/dm³.
• the formula mass is the sum of all the atomic masses
• molecules are made up of atoms joined together by covalent bonds
• ionic compounds have high melting points, low solubility in water and conduct electricity when molten or dissolved
• the relative formula mass is the sum of all the relative atomic masses
• What is the mole in chemistry?

  The mole is a unit of measurement for the number of particles in a substance.
• Why are chemical amounts measured in moles instead of actual masses?

  Because the actual masses of atoms, molecules, and ions are too small to be useful in calculations.
• What is the symbol for the mole?

  The symbol for the mole is mol.
• How is the mass of one mole of a substance determined if it is an element?

  The mass of one mole of an element is equal to its relative atomic mass (A_r) in grams.
• How is the mass of one mole of a substance determined if it is a compound?

  The mass of one mole of a compound is equal to its relative formula mass (M_r) in grams.
• What is the relationship between mass, moles, and relative formula mass?

  • The mass of a substance can be calculated from the number of moles.
  • The number of moles can be calculated from its mass.
  • The link between the two quantities is the relative formula mass.
• What is the formula to calculate the amount of a substance in moles?

  The formula is amount = mass / relative atomic or formula mass.
• How do balanced equations relate to moles?

  • A balanced equation shows the amounts in moles of reactants and products.
  • Coefficients in the equation indicate the amounts of reactants and products.
  • If there is no coefficient, the amount is one mole.
• How can the masses of substances in a balanced equation be calculated?

  The masses can be calculated using the equation: mass = relative formula mass x amount.
• What can be calculated using balanced equations and relative formula mass values?

  • The mass of product made from a given mass of reactant.
  • The mass of reactant needed to make a given mass of product.
• What is a limiting reactant in a chemical reaction?

  The limiting reactant is the reactant that is all used up in a reaction.
• What happens to the other reactant when the limiting reactant is used up?

  The other reactant has nothing left to react with, so some of it is left over.
• How does the mass of product formed in a reaction depend on the limiting reactant?

  The mass of product formed depends on the mass of the limiting reactant, as no more product can form when it is all used up.