C3 - quantitative chemistry

• what is the conservation of mass?

  the law that no atoms are lost or made during a chemical reaction so that the mass of products equals the mass of the reactants
• what is the relative formula mass of a compound?

  the sum of the relative atomic masses of the atoms in the numbers shown in the formula
• what does mass do when a reactant or products is a gas?

  changes
• what are chemical amount measured in?

  moles
• what is the Avogadro constant?

  the number of atoms, molecules or ions in a mole of a given substance, the value of Avogadro constant is 6.02 x 10^23 per mole
• what is the equation to find the number of particles?

  number of particles = number of moles x Avogadro's constant
• what does $Mg\ +$$\ 2HCl\ \rightarrow\ MgCl^2\ +$$H^2$ show us? 


  shows us the one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.
• what is the general method for reacting mass questions?

  1. work out the number of moles of the substance for which the mass has been given using number of moles=mass/Mr
  2. use the ratios of moles in the balanced equation to work out the moles of the other substance
  3. work out the mass of the second substance using mass = moles x Mr
• what is a limiting reactant?

  the reactant in a chemical reaction that is all used up because it limits the amount of products
• what does the number of moles of the limiting reactant determine?

  the number of moles of the product formed
• what is the general method for limiting reactant questions?

  1. work out the number of moles of the substance for each reactant using number of moles = mass/Mr
  2. use the ratios of moles in the balanced equation to work out which reactant is the limiting reactant
  3. use the ratio of moles in the balanced equation to convert the moles of the limiting reactant to the moles of the product
  4. work out the mass of the product using mass = moles x Mr
• example of limiting reactants question

  
• what is the concentration equation?

  concentration = moles / volume
• calculate the concentration of solution made by dissolving 5g of $Na_2CO_3$ in 250$cm^2$ water? 


  moles = mass/Mr

  = 5/(23 x 2 + 12 + 16 x 3)

  = 0.0472

  concentration = moles/volume

  = 0.0472 / 0.25

  = 0.189 mol/dm^3
• what is the yield?

  the amount of a product obtained
• what is the equation for percentage yield?

  $\frac{mass\ of\ product\ actually\ made}{\max imun\ theoretical\ mass\ of\ product\ }\times\ 100$
• 25.0g of $Fe_2O_3$ was reacted and it produced 10.0g of Fe. calculate the percentage yield?

  $Fe_2O_3+$$3CO\ \rightarrow 2Fe +$$3CO_2$

  work out moles of iron oxide,

  = 25.0/160,

  = 0.156 mol

  use balanced equation to give moles of Fe,

  = 1 moles $Fe_2O_3$ : 2 moles Fe

  = so 0.156 $Fe_2O_3$ : 0.313 Fe

  calculate maximum mass of Fe

  = mass=moles x Ar

  = 0.313 x 56

  = 17.5g

  calculate the percentage yield

  = (10/17.5) x 100

  = 57.1%
• what is percentage atom economy?

  a measure of the amount of starting materials that end up as useful products
• what is the formula for percentage atom economy?

  $\frac{relative\ formula\ mass\ of\ desired\ product\ from\ equation}{sum\ of\ relative\ formula\ masses\ of\ all\ reac\tan t\ from\ equation}\times100$
• calculate the % atom economy fro the following reaction where Fe is the desired product assuming the reaction goes to completion?

  $Fe_2O_3+$$3CO\ \rightarrow\ 2Fe+$$3CO_2$

  % atom economy = \frac{\left(2\times56\right)}{\left(2\times56+3\times16\right)+3\times\left(12+16\right)}\times100

  = 45.9%
• what is the method for the titration practical?

  • alkali in burette
  • acid in conical flask measured out with 25cm^3
  • few drops of indicator
  • add alkali to acid until colour changes
  • swirl conical flask
  • add alkali drop wise towards the end
  • note the final burette reading
  • repeat until two readings are withing 0.1cm^3
• what is the titrations calculations general method?

  1. calculate the number of moles of the substance for which the volume and concentration has been given using number of moles = concentration x volume
  2. use the balanced equation to work out the moles of the other substance
  3. calculate the concentration of the second substance using concentration = moles / volume
• what is the equation for gas volume?

  gas volume = number of moles x 24
• calculate the volume in dm^3 at room temperature and pressure of 50g of carbon dioxide gas.

  convert mass to moles

  moles = mass/Mr

  =50/(12+16 x 2)

  = 1.136 mol

  convert moles to gas volumes

  gas volume = moles x 24

  = 1.136 x 24

  = 27.3 dm^3
• what equation is used for finding pure solids and gases?

  moles = mass/Mr
• what equation is used to find solutions?

  concentration = moles / volumes
• what equation is used to find gases?

  gas volume = moles x 24