3 - Quantitative Chemistry

• Law of conservation of mass: no atoms are lost or made during a chemical reaction so the mass of the products equal the mass of the reactants 
• The charge on a metal ion is often the same as the group number in the periodic table. 

  • For example, sodium is in Group 1 and forms a 1+ ion, magnesium is in Group 2 and forms a 2+ ion
  • Transition metals can form several ions
• In an ionic compound the charges on the ions have to cancel out to leave an overall charge of 0

  
• Balancing Chemical Equations

  Never change small numbers- produces a new molecule, large numbers means you have multiple molecules

  
• Relative formula mass Mr: of a compound is the sum of the relative atomic mass of the atoms in the numbers shown in the formula 

  • Has no unit 
  • Never involves big numbers 

  Elements x Ar = Mr   Mr as the sum of the relative atomic masses

  
• Relative atomic mass is the average of the mass number of the different isotopes- is weighted of the abundance of each isotope (how common each isotope is) Ar (% of isotope b x mass of isotope a)+ (% of isotope b x mass of isotope b)/ 100 = Ar

  
•  % of mass = total relative atomic mass of element / relative formula mass of compound (Mr)

  
• number of moles = mass / relative atomic mass (Ar)

  (Ar = bottom number on element)

  
• Mass (g) = number of moles x Relative formula mass (Mr)

  
• Avogadro's constant 6.02x1023

  1. Calculate number of moles (mass/Ar)
  2. Multiple number of molecules (each element) by 6.02 x 1023

  
• using moles to balance questions

  1. calculate relative atomic mass (bottom number on elements)
  2. substitute numbers in and calculate the moles

  
• Reacting Masses

  1. Look at big numbers in the reaction- tells us how many moles there are
  2. calculate moles - number of moles = mass/Relative formula mass (Mr)
  3. Divide moles, based off of original moles
  4. Find mass - Mass=moles x Mr

  
• Relative formula mass = [number of element x atomic mass]
• Limiting Reactants

  1. write out info that you're given- eg: number of moles, the least limited mole is the smallest number
  2. calculate the mass- mass=moles x Mr

  
• Concentration tells us the mass of a solute in a given volume of solution

  • A solute is a dissolved chemical, water is often a solvent (in chem) 
  • Unit of concentration g/dm3

  concentration = mass/volume

  
• Limiting reaction = the reactant that is all used up it sets a limit on how much product can form.

  The reactant that is left over is described as being in excess