Quantitative chemistry

• What does the law of conservation of mass state?

  No atoms are lost or made during a chemical reaction.
• How does the law of conservation of mass relate to chemical reactions?

  The mass of the products equals the mass of the reactants.
• What does it mean for chemical reactions to be represented by symbol equations?

  They are balanced in terms of the numbers of atoms of each element on both sides.
• What should students understand about multipliers in equations?

  They are used in normal script before a formula and in subscript within a formula.
• What is the relative formula mass (M_r) of a compound?

  It is the sum of the relative atomic masses of the atoms in the formula.
• What is true about the sum of relative formula masses in a balanced chemical equation?

  The sum of the relative formula masses of the reactants equals that of the products.
• How can students calculate the percentage by mass in a compound?

  By using the relative formula mass and the relative atomic masses.
• Why might some reactions appear to involve a change in mass?

  Because a reactant or product is a gas and its mass has not been accounted for.
• What happens when a metal reacts with oxygen?

  The mass of the oxide produced is greater than the mass of the metal.
• What occurs during the thermal decomposition of metal carbonates?

  Carbon dioxide is produced and escapes, leaving the metal oxide as the only solid product.
• How should students explain observed changes in mass in non-enclosed systems during a chemical reaction?

  By using the balanced symbol equation and the particle model.
• What is always present when a measurement is made?

  Some uncertainty about the result obtained.
• How can students represent the distribution of results in measurements?

  By making estimations of uncertainty.
• What does the range of a set of measurements indicate?

  It serves as a measure of uncertainty about the mean.
• What unit is used to measure chemical amounts?

  Moles, symbolized as mol.
• How is the mass of one mole of a substance related to its relative formula mass?

  It is numerically equal to its relative formula mass in grams.
• What does one mole of a substance contain?

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

  1. 02 x 10²³ per mole.
• How does the measurement of amounts in moles apply to carbon?

  In one mole of carbon (C), the number of atoms is the same as the number of molecules in one mole of carbon dioxide (CO₂).
• How can students calculate the number of moles in a given mass of a substance?

  By using the relative formula mass of the substance.
• What can be calculated from balanced symbol equations?

  The masses of reactants and products.
• How is the following equation interpreted: $Mg +$$2HCl \rightarrow MgCl_2 +$$H_2$?


  One mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.
• How can students calculate the masses of substances shown in a balanced symbol equation?

  By using the balanced symbol equation and the mass of a given reactant or product.
• 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 the limiting reactant in a chemical reaction?

  The reactant that is completely used up, limiting the amount of products formed.
• How does the concentration of a solution relate to its mass and volume?

  It can be measured in mass per given volume, such as grams per dm³.
• How can students calculate the mass of solute in a given volume of solution?

  By knowing the concentration of the solution in terms of mass per given volume.
• What is the percentage yield of a product?

  It is the amount of product obtained compared to the maximum theoretical amount.
• Why might the actual yield of a product be less than the theoretical yield?

  Because the reaction may not go to completion or some product may be lost.
• How is percentage yield calculated?

  Using the formula: $\% \text{Yield} =$$\frac{\text{Mass of product actually made}}{\text{Maximum theoretical mass of product}} \times 100$
• How can students calculate the atom economy of a reaction?

  By using the balanced equation for the reaction.
• What is atom economy?

  It is a measure of the amount of starting materials that end up as useful products.
• How is the percentage atom economy calculated?

  Using the formula: $\text{Percentage Atom Economy} =$$\frac{\text{Relative formula mass of desired product}}{\text{Sum of relative formula masses of all reactants}} \times 100$
• How is the concentration of a solution measured?

  In mol/dm³.
• How can the amount in moles of solute be calculated from its concentration?

  By using the concentration in mol/dm³ and the volume of the solution.
• What is true about equal amounts in moles of gases?

  They occupy the same volume under the same conditions of temperature and pressure.
• What is the volume of one mole of any gas at room temperature and pressure?

  24 dm³.
• How can students calculate the volume of a gas at room temperature and pressure?

  From its mass and relative formula mass.
• How can volumes of gaseous reactants and products be calculated?

  From a balanced equation and a given volume of a gaseous reactant or product.