1.3

• What is the molecular formula?


  The actual number of atoms of each element present in the molecule
• What is the empirical formula?


  The simplest whole number ratio/proportion of elements present
• How do you determine the number of units in a molecule from the empirical formula mass?


  Divide the molar mass by the mass of the empirical formula
• What is the molar gas volume at standard temperature and pressure (STP)?


  22.4 dm³
• What volume of hydrogen is formed at STP when 6.00 g of zinc reacts with excess sulfuric acid?


  1. 061 dm³
• How do you calculate the moles of zinc from its mass of 6.00 g?


  $\text{moles zinc} =$$\frac{6.00}{65.4} =$$0.0920 \text{ mol}$
• What is the mole ratio of zinc to hydrogen in the reaction with sulfuric acid?


  1 mol Zn : 1 mol H₂
• How do you calculate the volume of hydrogen gas produced from 0.0920 mol at STP?


  $\text{volume of hydrogen} =$$0.0920 \times 22.4 =$$2.061 \text{ dm}^3$
• What is the molar gas volume at room temperature and pressure (RTP)?


  24.5 dm³
• What does the equation $\frac{p_1 V_1}{T_1} =$$\frac{p_2 V_2}{T_2}$ represent?


  It shows how changes in pressure, volume, and temperature affect a gas
• How do you convert temperature from Celsius to Kelvin for gas calculations?


  Add 273 to the Celsius temperature
• What are the characteristics of an ideal gas?


  • Particles are of negligible size
  • Move with rapid, random motion
  • Collisions are perfectly elastic
  • No intermolecular forces
• What is the ideal gas equation?


  $pV =$$nRT$
• What units must be used in the ideal gas equation?

  • Pressure: Pa or Nm⁻²
  • Volume: m³
  • Moles: mol
  • R: 8.31 J mol⁻¹ K⁻¹
  • Temperature: K
• How do you convert 200 cm³ of gas to m³ for calculations?


  $200 \text{ cm}^3 =$$200 \times 10^{-6} \text{ m}^3$
• How do you calculate the number of moles in 200 cm³ of gas at 25°C and 100 kPa?


  $n =$$\frac{(100 \times 10^3) \times (200 \times 10^{-6})}{8.31 \times 298} =$$0.808 \text{ mol}$
• What is the formula to calculate moles from concentration and volume?


  $\text{moles} =$$\text{volume (dm}^{-3}) \times \text{concentration (mol dm}^{-3})$
• How do you calculate the concentration of sodium hydroxide from a titration with hydrochloric acid?


  $\text{concentration of NaOH} =$$\frac{0.0125 \text{ mol}}{0.035 \text{ dm}^3} =$$0.357 \text{ mol dm}^{-3}$
• What is the formula for calculating atom economy?


  $\text{atom economy} =$$\frac{\text{mass of required product}}{\text{total mass of reactants}} \times 100$
• What is the formula for calculating percentage yield?


  $\text{percentage yield} =$$\frac{\text{mass of product obtained}}{\text{maximum theoretical mass}} \times 100$
• How is percentage error calculated when measuring a solution from a burette?


  $\text{percentage error} =$$\frac{\text{error}}{\text{measured value}} \times 100$
• What is the percentage error when measuring 24.30 cm³ from a burette with an error of 0.05 cm³ per reading?


  $\text{percentage error} =$$\frac{0.1}{24.30} \times 100 =$$0.41\%$
• How does the percentage error change with the amount measured?


  Percentage errors decrease as the amount measured increases
• What are the steps to calculate the empirical and molecular formula from a hydrocarbon's mass composition?


  1. Determine the mass of each element.
  2. Calculate the moles of each element.
  3. Find the simplest ratio of moles.
  4. Write the empirical formula.
  5. Calculate the molar mass of the empirical formula.
  6. Divide the molar mass of the compound by the empirical formula mass to find the molecular formula.
• What are the conversions needed for gas calculations?

  • 1 kPa = 1 × 10³ Pa
  • 1 atm = 1.01 × 10⁵ Pa
  • 1 cm³ = 1 × 10⁻⁶ m³
  • 1 dm³ = 1 × 10⁻³ m³
• What are the key points regarding significant figures in measurements?


  • A zero does not count as a significant figure until it has a non-zero number in front of it.
  • The overall error is the sum of individual errors in measurements.
  • Larger amounts measured decrease the percentage error.
• What is relative atomic mass (Ar)?


  The average mass of one atom of the element relative to one-twelfth the mass of an atom of carbon-12
• Why is an average mass used for relative atomic mass?


  Because most elements exist as two or more different isotopes
• Does relative atomic mass have units?


  No, it has no units as the masses are relative to another mass
• What is relative isotopic mass?


  The mass of one atom of an isotope relative to one-twelfth the mass of one atom of carbon-12
• What is relative formula mass (Mr)?


  The total average mass of all the atoms in the formula relative to one-twelfth the mass of an atom of carbon-12
• How can a mass spectrometer be used?


  To find the relative atomic mass of an element
• What are the four main processes in a mass spectrometer?


  1. Ionisation
  2. Acceleration
  3. Deflection
  4. Detection