cognito #26-49

• Conservation of mass

  In a chemical reaction, mass is always conserved; no atoms are created or destroyed, only the bonds between atoms change
• Balancing a chemical equation

  1. Identify reactants

  2. Count atoms

  3. Adjust coefficients

  4. Ensure both sides balance
• The mass should stay the same throughout the entire reaction when measured on scales
• Exception to conservation of mass

  When a reaction involves a gas, the mass might seem to change because the gas can escape into the air
• If the experiment is done in a sealed container, the mass of reactants will balance perfectly with the products
• The gaseous CO2 would float off into the air, making the weight of products seem decreased
• In a sealed container, the mass would stay the same because all the CO2 would be trapped
• Reaction of hydrochloric acid and calcium carbonate

  1. Drop calcium carbonate in hydrochloric acid

  2. Observe solid disappearing

  3. Observe bubbles of gas

  4. Identify limiting reactant when reaction stops
• Limiting reactant

  The reactant that is completely used up in a reaction, limiting the amount of product produced
• Excess reactant

  The reactant that remains after the reaction has completed
• The amount of product we get depends entirely on the limiting reactant
• If we add more calcium carbonate

  More fizzing occurs until it is used up
• If we add more hydrochloric acid

  Nothing will happen if calcium carbonate is not present
• Burning in air means reacting with oxygen
• Calculating mass of sodium oxide produced

  1. Write out the equation

  2. Balance the equation

  3. Calculate moles of sodium

  4. Find ratio of sodium to sodium oxide

  5. Calculate mass of sodium oxide
• Moles

  A measure of the amount of substance
• The equation for calculating moles is moles equals mass over mr
• Finding mass of sodium oxide

  1. Rearrange equation to mass equals moles times mr

  2. Calculate mr of sodium oxide

  3. Plug in values to find mass
• Finding volume from moles

  Multiply moles by 24
• Finding moles from volume

  Volume divided by 24
• Finding volume from mass

  1. Find moles first

  2. Divide mass by relative formula mass

  3. Multiply moles by 24
• Relative formula mass (mr)


  Sum of atomic masses in a molecule
• Finding volume of product from reactant

  1. Find moles of reactant

  2. Use molar ratio

  3. Multiply by 24
• Molar ratio

  Ratio of moles of reactants and products in a chemical reaction
• The equation with the number 24 only works for gases at room temperature and pressure
• Gases occupy different volumes at different temperatures or pressures
• Number of moles

  Links to concentration and volume
• Volume

  Measured in decimeters cubed
• Concentration

  Measured in moles per decimeter cubed
• Concentration is normally written as moles per decimeter cubed
• Finding concentration

  Concentration = moles / volume
• Volumes must be in decimeters cubed for calculations
• Steps to find unknown concentration


  1. Calculate moles of known substance

  2. Use molar ratio

  3. Calculate concentration of unknown substance
• Molar ratio

  Ratio of moles between substances in a reaction
• Calculating concentration of sulfuric acid

  Concentration = moles / volume
• Decimeter cubed

  A measurement of volume equal to 1,000 centimeters cubed and the same size as one liter of water
• Concentration

  Mass divided by volume
• Calculating concentration

  Mass divided by volume
• Finding mass using formula triangle

  Concentration times volume
• Finding volume using formula triangle

  Mass divided by concentration