3. Quantitative Chemistry

• The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in the chemical formula
• To calculate the percentage mass of an element in a compound, you multiply the Ar of the element by the number of atoms of that element, divide by the Mr of the whole compound, and multiply by 100
• The law of conservation of mass states that no atoms are created or destroyed in a chemical reaction, so the mass of the reactants equals the mass of the products
• Mass can seem to change during a reaction due to gases being formed or absorbed, but the total mass is conserved
• Random errors


  Measured values vary randomly around the true value, due to human error and random variations
• Systematic errors


  Issues in experimental design or equipment cause the measured value to be consistently too high or too low
• Uncertainty


  A number that tells you how much your results might be off by
• Analogue instruments


  Uncertainty is usually half of the smallest thing you can measure on it
• Digital instruments


  Uncertainty is just the smallest number it can display
• Mole


  A measurement for the amount of substance of a chemical, with one mole having the same number of particles as Avogadro's constant (6.02 x 10^23)
• One mole of any substance has the same mass in grams as its own relative formula mass (Mr)
• Calculating moles from mass


  Divide the mass by the compound's Mr
• Calculating mass from moles

  Multiply the moles by the Mr
• Using moles in chemical equations

  Find the moles of the known substance, use the ratio in the balanced equation to find the moles of the unknown, then calculate the mass of the unknown
• Using moles to balance equations


  Find any unknown masses, find the Mr for each reactant and product, divide the masses by the Mr to find the moles, divide each mole by the smallest number to get whole numbers, and put these numbers in front of the chemical formulas
• Limiting reactant


  The substance that is completely used up before the excess reactant, determining the amount of product that can be formed
• Determining the limiting reactant

  Find the Mr of each reactant, convert the mass of each to moles, and compare the moles to the ratio in the balanced equation
• Limiting reactant


  The reactant that is used up before the excess reactant. When the limiting reactant gets used up, the reaction stops meaning the excess reactant would remain without reacting. The amount of product that can be formed depends on the amount of limiting reactant.
• When performing reacting mass calculations, the limiting reagent is always the number that should be used, as it indicates the maximum possible amount of product that can form.
• Determining the limiting reactant

  1. Find the Mr of each reactant

  2. Convert the mass of each reactant into moles by dividing by the Mr

  3. Compare the moles with the balanced equation to see whether the moles match the ratio
• Solute


  A solid substance that dissolves in a liquid
• Solvent


  The liquid a solute dissolves in
• Solution


  The mixture that a solute and solvent form
• Concentration


  The amount of a substance within a certain volume of solution
• The more solute there is in a given volume, the more concentrated the solution is
• Concentration


  Typically measured in grams per decimetre cubed (g/dm³)
• Calculating mass of solute


  If you know the concentration and the volume, you can calculate the mass of solute using the rearranged formula
• Reactivity series


  Helps predict how different metals will react with a variety of substances, including acids and water
• Reactivity series


  • Ranks metals from the most reactive to the least based on how easily they lose electrons and form positive ions (cations)
  • The more easily a metal loses electrons and forms positive ions, the more reactive it is
• Highly reactive metals like potassium (K), sodium (Na), and lithium (Li) react the most vigorously with water and acid, producing hydrogen gas. This reaction can be explosive with these metals due to their high reactivity.
• Metals like magnesium (Mg) and calcium (Ca) also react with water and acid but less vigorously than the metals higher up in the series.
• Zinc (Zn) and iron (Fe) do not react with liquid water, instead the water needs to be in the form of steam in order to react. Which shows that they have an even lower reactivity. They react with acid very slowly in comparison to the metals above.
• Copper (Cu), is the only one that does not react with water OR acids, showing that it is at the bottom of this reactivity series.
• Metals combine with oxygen to form metal oxides. Most metals are found as metal oxides in the environment in rocks called ores, so they require chemical reactions to extract the pure metal from it.
• Oxidation


  When metals like Iron or Aluminum react with Oxygen, they form metal oxides. This is known as oxidation, a chemical process involving the gain of oxygen or the loss of electrons.
• Reduction


  The process of extracting a metal from its oxide is known as a reduction reaction, because it involves the loss of oxygen or the gain of electrons.
• Extraction techniques


  • No extraction (for unreactive metals like gold)
  • Reduction with carbon (for metals less reactive than carbon)
  • Electrolysis (for metals more reactive than carbon)
• Redox


  Stands for reduction-oxidation. It is when reduction AND oxidation occur in the same reaction. These reactions are all about the transfer of electrons between substances.
• Oxidation


  The process where a substance loses electrons. This often involves the addition of oxygen.
• Reduction


  The process where a substance gains electrons, typically losing oxygen.