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 must be true in a balanced chemical equation regarding the sum of Mr of reactants and products?

  The sum of Mr of reactants equals the sum of Mr of products.
• What happens to mass when a reactant or product is a gas?

  Mass may appear to change if the gas is not accounted for.
• What should you check if a reaction appears to involve a change in mass?

  Check if a reactant or product is a gas that has not been accounted for.
• What is an example of a reaction where mass of the product is greater than the mass of the reactant?

  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 uncertainty in measurements?

  Be prepared to make estimations of uncertainty.