Chemical Measurements

• What does the law of conservation of mass state?

  No atoms are lost or made during a chemical reaction.
• What does it mean for a chemical equation to be balanced?

  It means the number of atoms of each element is the same 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 formula.
• What is the relative formula mass (Mr) of HCl?

  $Mr =$$1 +$$35.5 =$$36.5$
• What is the relationship between the sum of Mr of reactants and products in a balanced chemical equation?

  • Sum of Mr of reactants in quantities shown = Sum of Mr of products in quantities shown
  • No mass is lost during a reaction
• What should you check if a reaction appears to involve a change in mass?

  You should check if a reactant or product is a gas and its mass has not been accounted for.
• What happens to the mass when a metal reacts with oxygen?

  The mass of the metal oxide product is greater than the mass of the metal.
• What is always present when a measurement is made?

  There is always some uncertainty about the result obtained.
• What should you be prepared to do regarding measurements and uncertainty?

  You should be prepared to make estimations of uncertainty.