Chemical calculations with mass

• Conservation of mass


  Atoms cannot be created or destroyed in a chemical reaction. Atoms are rearranged into new substances. All the atoms you had in the reactants must be present in the products.
• When measuring the mass of a reaction, you would expect the mass at the start to be the same as the mass at the end. However, sometimes the mass can appear to change.
• Decrease in mass


  Normally because a gas is produced in the reaction and lost to the surroundings
• Decrease in mass reaction

  • sodium + water → sodium hydroxide + hydrogen
• Balancing symbol equations


  Write the unbalanced equation

  2. Balance the number of atoms on each side
• Formula mass


  The relative atomic mass of all the atoms in the formula
• Avogadro's constant


  One mole of a substance contains 6.00x10^23 atoms, ions, or molecules
• One mole of a substance has the same mass as the formula mass of the substance
• State symbols


  Indicate the physical state of a substance in a chemical reaction
• Increase in mass


  Normally because one of the reactants is a gas
• Increase in mass reaction

  • sodium + chlorine → sodium chloride
• Excess reactant


  The reactant that is left over after the reaction
• Limiting reactant


  The reactant that runs out first in the reaction
• Determining limiting and excess reactants

  Write the balanced symbol equation

  2. Pick one reactant and its quantity

  3. Use the ratio in the balanced equation to calculate the amount of the other reactant needed

  4. Compare this to the quantity given

  5. Determine which is excess and which is limiting
• Concentration


  The amount of solute in a volume of solvent
• Calculating concentration


  concentration (g/dm³) = mass (g) / volume (dm³)
• To convert concentration from mol/dm³ to g/dm³, multiply by the molar mass (Mr)
• What is the conservation of mass?

  In a chemical reaction, atoms are not created or destroyed, just rearranged, so total mass before = total mass after the reaction
• When a metal forms a metal oxide, why does the mass increase?

  Atoms from gaseous oxygen have been added
• When an acid reacts with a metal, why does the mass decrease?

  A gas is produced and escapes
• What is the relative formula mass?

  the sum of the relative atomic masses of each atom in a substance
• What are the four state symbols and what do they stand for?

  (s) solid, (l) liquid, (g) gas, (aq) aqueous or dissolved in water
• How can you tell when a symbol equation is balanced?

  the number of atoms of each element is the same on both sides
• What is a mole?


  mass of a substance that contains 6.02x10^23 particles
• Give the value for Avogadro's constant.

  6.02x10^23
• Which formula is used to calculate the number of moles from mass and Mr?

  moles=mass/Mr
• Which formula is used to calculate the mass of a substance from number of moles and Mr?

  mass=molesxMr
• What is a limiting reactant?

  the reactant that is completely used up in a chemical reaction.
• What is a unit for concentration?

  g/dm^3 or mol/dm^3
• Which formula s used to calculate concentration from mass and volume?

  concentration (g/dm^3)=mass(g)/volume(g/dm^3)
• Which is formula is used to calculate volume from concentration and mass?

  volume (dm^3)=mass(g)/concentration(g/dm^3)
• Which formula is used to calculate mass from concentration in g/dm^3 and volume?


  mass (g)=concentration(g/dm^3) x volume(dm^3)
• How can you convert a volume reading in cm^3 to dm^3?

  divide by 1000
• If the amount of solute in a solution is increased, what happens to its concentration?

  increases
• If the volume of water in a solution is increased, what happens to its concentration?

  decreases