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 (Mr) 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.
• Why might a reaction appear to involve a change in mass?

  This could be 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 the unit for measuring chemical amounts?

  The unit is moles, symbolized as mol.
• How does the mass of one mole of a substance relate to its relative formula mass?

  The mass of one mole in grams is numerically equal to its relative formula mass.
• How many grams does one mole of iron weigh if its Mr is 56?

  56 grams.
• How many grams does one mole of nitrogen gas (N₂) weigh if its Mr is 28?

  28 grams.
• What is the Avogadro constant?

  1. 02 x $10^{23}$ per mole.
• How can chemical equations be interpreted in terms of moles?

  They show the ratio of moles of reactants and products involved in the reaction.
• In the equation Mg + 2HCl → MgCl₂ + H₂, how many moles of HCl react with 1 mole of Mg?

  2 moles of HCl.
• What must be true about the total moles of one element in a balanced equation?

  The total moles must be the same on both sides of the equation.
• What is the first step to balance a chemical equation using masses?

  Convert the masses in grams to amounts in moles.
• What is a limiting reactant?

  The limiting reactant is the reactant that is used up and limits the amount of products formed.
• How does the amount of product produced relate to the limiting reactant?

  The amount of product produced is restricted to the amount of the limiting reactant that reacts.
• How can concentration of a solution 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 equation mass = concentration x volume.
• 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.
• How is percentage yield calculated?

  Percentage yield = (Amount of product produced / Maximum amount of product possible) x 100.
• Why might the actual yield of a product be less than the theoretical yield?

  Because the reaction may not go to completion, some product may be lost, or reactants may react differently than expected.
• How do you calculate the theoretical mass of a product from a given mass of reactant?

  Calculate moles of reactant, use balancing numbers to find moles of product, then calculate theoretical mass using mass = moles x molar mass.
• What is atom economy?

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

  Atom economy = (Mr of desired product / sum of Mr of all reactants) x 100.
• What factors might influence the choice of a particular reaction pathway?

  Factors include atom economy, yield, rate, equilibrium position, and usefulness of by-products.
• How can concentration of a solution also be measured?

  In moles per given volume of solution, e.g., mol per dm³ (mol/dm³).
• How are mass of a solute and volume of a solution related to concentration?

  They are related through the equation moles = concentration x volume.
• How do you calculate the concentration of a solution if the volumes of two solutions that react completely are known?

  Work out moles of the known solution, then use mole ratio from the equation to find moles of the other solution, and finally calculate concentration.
• What is the volume of 1 mole of any gas at room temperature and pressure (RTP)?

  24 dm³.
• How can you calculate the volume of gas produced at RTP?

  Use the equation Volume of gas (dm³) at RTP = Moles x 24.
• If you produce 5 moles of hydrogen gas, what is the volume produced at RTP?

  120 dm³.