Calculations in Chemistry

• What is used instead of atomic mass for atoms?

  Relative atomic masses
• What does the relative atomic mass of an element represent?

  Mass of its atoms compared to carbon-12
• What is the symbol for relative atomic mass?

  Ar
• Where can you find the Ar values for elements?

  In the periodic table
• Does relative atomic mass have units?

  No, it has no units
• What is the relative formula mass of a substance made up of?

  Sum of relative atomic masses of its atoms
• What is the symbol for relative formula mass?

  Mr
• How do you calculate the Mr for a substance?

  Sum Ar values of all atoms in formula
• What is the formula for carbon dioxide?

  CO2
• How many carbon and oxygen atoms are in CO2?

  1 carbon and 2 oxygen atoms
• How is the Mr of CO2 calculated?

  12 + 16 + 16 = 44
• What is another way to calculate the Mr of CO2?

  (1 × 12) + (2 × 16) = 44
• Do Mr values have units?

  No, they are relative masses
• How are relative formula masses of ionic compounds calculated?

  Same way as molecular substances
• Do ionic compounds exist as molecules?

  No, they do not exist as molecules
• What are the steps to calculate relative formula mass (Mr)?

  1. Identify the number of atoms of each element.
  2. Find the Ar values for each element.
  3. Sum the Ar values based on the number of atoms.
• What is the difference between relative atomic mass (Ar) and relative formula mass (Mr)?

  • Ar: Mass of individual atoms compared to carbon-12
  • Mr: Sum of Ar values for all atoms in a formula
• What does the law of conservation of mass state?

  No atoms are lost or made in reactions
• How do atoms behave in a chemical reaction according to the law of conservation of mass?

  Atoms join together in different ways
• Why is the number of atoms the same on both sides of a balanced symbol equation?

  Atoms are neither lost nor made
• What is the relationship between the total mass of products and reactants?

  Total mass of products equals total mass of reactants
• What is equal to the sum of the relative formula masses of the reactants?

  Sum of the relative formula masses of products
• How can the mass of one substance in a reaction be calculated?

  If the masses of other substances are known
• What happens to substances in a closed system during a reaction?

  No substances can enter or leave
• What is an example of a closed system in a reaction?

  A stoppered flask
• How do acid-alkali neutralization reactions behave in terms of mass?

  They can act as closed systems
• What happens in non-enclosed systems during reactions?

  Substances can enter or leave
• What happens when copper carbonate decomposes on heating?

  It produces solid copper oxide and carbon dioxide
• What occurs when magnesium reacts with oxygen?

  Magnesium oxide is produced from oxygen
• What effect does escaping gas have on total mass in a reaction?

  Total mass appears to decrease
• What happens to total mass if a gas is gained in a reaction?

  Total mass appears to increase
• What must be included to maintain the total mass in a reaction?

  The mass of the gas must be included
• What is always present in chemical measurements?

  Uncertainty
• What are two examples of uncertainty in measurements?

  Judging thermometer readings and reaction completion
• How can uncertainty be estimated from measuring instruments?

  By considering the resolution of instruments
• What is the resolution of a measuring instrument?

  Smallest change in quantity that can be seen
• What is the resolution of a thermometer with marks every 1.0°C?


  1.0°C
• How is uncertainty estimated for a thermometer with a mark at every 1.0°C?

  ± 0.5°C
• If a thermometer reads 24.0°C, how can the result be expressed with uncertainty?

  24.0°C ± 0.5°C
• How is uncertainty determined for a digital measuring instrument?

  Half the last digit shown on display