Reaction Calculations (OCR A Level Chemistry)

• The number of moles of a substance can be found by using the following equation:

• It is important to be clear about the type of particle you are referring to when dealing with moles
  • E.g. one mole of CaF₂ contains one mole of CaF₂ formula units, but one mole of Ca²⁺ and two moles of F^- ions

Reacting masses

• The masses of reactants are useful to determine how much of the reactants exactly react with each other to prevent waste
• To calculate the reacting masses, the balanced chemical equation is required
• This equation shows the ratio of moles of all the reactants and products, also called the stoichiometry, of the equation
• To find the mass of products formed in a reaction the following pieces of information are needed:
  • The mass of the reactants
  • The molar mass of the reactants
  • The balanced equation

Answer

Step 1: The balanced symbol equation is:

Step 2: The relative formula masses are:

Step 3: Calculate the moles of magnesium used in the reaction:

= 0.25 moles

Step 4: Find the ratio of magnesium to magnesium oxide using the balanced chemical equation

Therefore, 0.25 mol of MgO is formed

Step 5: Find the mass of magnesium oxide

Therefore, the mass of magnesium oxide produced is 10 g

Stoichiometric relationships

• The stoichiometry of a reaction can be found if the exact amounts of reactants and products formed are known
• The amounts can be found by using the following equation:

• The gas volumes can be used to deduce the stoichiometry of a reaction
  • E.g. in the combustion of 50 cm³ of propane reacting with 250 cm³ of oxygen, 150 cm³  of carbon dioxide is formed suggesting that the ratio of propane : oxygen : carbon dioxide is 1 : 5 : 3

C₃H₈ (g) + 5O₂ (g) → 3CO₂ (g) + 4H₂O (l)

• The concentration of a solution is the amount of solute dissolved in a solvent to make 1 dm³ of  solution
  • The solute is the substance that dissolves in a solvent to form a solution
  • The solvent is often water

• A concentrated solution is a solution that has a high concentration of solute
• A dilute solution is a solution with a low concentration of solute
• When carrying out calculations involve concentrations in mol dm^-3 the following points need to be considered:
  • Change mass in grams to moles
  • Change cm³ to dm³ 

• To calculate the mass of a substance present in solution of known concentration and volume:
  • Rearrange the concentration equation

number of moles (mol) = concentration (mol dm^-3) x volume (dm³)

• • Multiply the moles of solute by its molar mass

mass of solute (g) = number of moles (mol) x molar mass (g mol^-1)

Answer

Step 1: Write the balanced symbol equation

CaCO₃  +  2HCl  →  CaCl₂  +  H₂O  +  CO₂

Step 2: Calculate the amount, in moles, of calcium carbonate that reacts

Step 3: Calculate the moles of hydrochloric acid required using the reaction’s stoichiometry

1 mol of CaCO₃ requires 2 mol of HCl

So 0.025 mol of CaCO₃ requires 0.05 mol of HCl

Step 4: Calculate the volume of HCl required

Volume of hydrochloric acid = 0.05 dm³

Answer

Step 1: Write the balanced symbol equation

Na₂CO₃  +  2HCl  →  2NaCl  +  H₂O  +  CO₂

Step 2: Calculate the amount, in moles, of sodium carbonate reacted by rearranging the equation for amount of substance (mol) and dividing the volume by 1000 to convert cm³ to dm³

amount (Na₂CO₃) = 0.025 dm³ x 0.050 mol dm^-3 = 0.00125 mol

Step 3: Calculate the moles of hydrochloric acid required using the reaction’s stoichiometry

1 mol of Na₂CO₃ reacts with 2 mol of HCl, so the molar ratio is 1 : 2

Therefore 0.00125 moles of Na₂CO₃ react with 0.00250 moles of HCl

Step 4: Calculate the concentration, in mol dm^-3, of hydrochloric acid

Volumes of gases

• Avogadro suggested that ‘equal volumes of gases contain the same number of molecules’ (also called Avogadro’s hypothesis)
• At room temperature (20 degrees Celsius) and pressure (1 atm) one mole of any gas has a volume of 24.0 dm³
• This molar gas volume of 24.0 dm³ mol^-1 can be used  to find:
  • The volume of a given mass or number of moles of gas:

volume of gas (dm³) = amount of gas (mol) x 24 dm³ mol^-1

• The mass or number of moles of a given volume of gas:

Answer