Calculations

• Titration Salt Prep (soluble salts from soluble bases)

  1. wash burette, pipette, and conical flask.
  2. add acid to 0.00 mark in burette and add 25.0cm^3 of alkali to conical flask.
  3. add named indicator to conical flask, then place on top off white tile.
  4. add acid dropwise whilst swirling until colour change.
  5. repeat until concordant results (within 0.1 (maybe 0.2 depends on ms)).
  6. calculate the mean value of concordant results only.
• avagadros number (L) - the number of units in one mole of a substance. L = 6.022 x 10^23

  The number of any specified particle (atoms/electrons/molecules) can be calculated using the following equation.

  number of particles = n x L
• avagadros number questions examples

  
  

  calculate the number of H2O molecules in 0.5 mol of H2O

  (0.5) x (6.022 x 10^23) = 3.011 x 10^23

  
  

  calculate the number of hydrogen atoms in 24g methanol.

  n(methanol) = 24/32 = 0.75

  (0.75) x (6.022 x 10^23) = 4.5165 x 10^23

  each methanol has 4 hydrogens:

  (4.5165 x 10^23) x 4 = 1.8 x 10 ^24
• percentage yield = (actual yield/theoretical yield) x 100

  actual yield = from the experiment

  theoretical yield = from the calculation
• % atom economy is a measure of the number of atoms wasted in making a chemical. The higher the economy, the less waste.
• % atom economy = (Mr of desired product/sum of Mrs of products) x 100

  When calculating Mrs in atom economy, you must consider the balancing numbers.
• 1m = 10dm = 100cm

  1m^3 = 1000 dm^3 = 1000000 cm^3
• ideal gas calculation equation

  pv =nRT (*when doing this calculation, your answer will always come out with the ideal unit, so be careful to convert)

  
  

  pressure in pascals

  volume in m^3

  temperature in kelvin

  gas constant (R) is 8.314
• ideal gas calculations common conversions:

  degrees C to kelvin = +273

  dm^3 to m^3 = /1000

  cm^ to m^ = /1 million

  KPa to Pa = x 1000

  MPa to Pa = x 1 million
• number of moles = volume x concentration

  n = v x c
• number of moles = m/Mr or Ar

  
  

  n = m/Mr or Ar
• number of moles = volume in cm^3/24,000
• number of moles = volume in dm^3/24
• standard solution preparation from a specified mass of solid:

  1. weigh by difference an exact mass of solid. Do this by recording the mass of solid + weighing boat, then reweighing the weighing boat + subtracting the masses.
  2. dissolve the solid in distilled water, and transfer the liquid to a volumetric flask, using the funnel.
  3. Rinse the beaker that the solid had been dissolved in (use distilled water) and add the washings to the flask.
  4. make the solution up to 250cm^3 and invert 10 times to mix.
• % uncertainty = (instrument uncertainty/measurement) x 100

  25.0cm^3 pipette = 0.05cm^3 uncertainty (plus or minus)

  50.0cm^3 burette = 0.05cm^3 uncertainty (plus or minus)

  250.0 cm^3 standard flask = 0.05cm^3 uncertainty (plus or minus)

  top-pan balance = 0.01 grams uncertainty (plus or minus)
• limiting reagent - reactant that is not is excess.

  The amount of product formed in a reaction is dictated by the limiting reagent.
• empirical formula calculation steps

  1. mass or %
  2. divide by the Ar/Mr which gives your moles
  3. divide by the smallest
  4. you have your ratio (if numbers are decimals multiply to whole numbers)

  water of crystallisation is the same, but you compare your compound with water.
• molecular formula from empirical formula:

  multiplication factor = Mr of compound/Mr of empirical formula.

  molecular formula = multiplication factor × empirical formula.

  e.g. C4H5 is the empirical formula of a compound. Relative molecular mass of C4H5 is 106. Mr of C4H5 = 53

  106/53 = 2

  2 x C4H5 = C8H10
• special water of crystallisation example

  A) CaSO4

  B) xH2O

  C) H2O

  
• CORE PRACTICAL 1 - measuring the molar volume of a gas:

  there are 2 main methods of measuring the molar volume of a gas (method 1 = gas syringe)

  
• CORE PRACTICAL 1 - measuring the molar volume of a gas:

  there are 2 main methods of measuring the molar volume of a gas (method 1 = displacement of water)

  • The delivery tube set up is airtight so no gas is lost
  • The reaction does go to completion

  
• % error between 2 values:

  $\frac{EXPERIMENTAL\ VALUE\ -THEORETICAL\ VALUE}{THEORETICAL\ VALUE}\ \times100$
• accuracy - how close your experiment is to the theoretical value

  
  

  precision - how consistent your results are

  hence an experiment can be precise and not accurate or vice versa.

  Similarly greater precision doesn't yield better results if the results aren't accurate
• equation to calculate the mass of an element in a compound (when you have the mass of the compound)

  
  

  $m\left(element\right)=$$m\left(compound\right)\times\left(\frac{Ar\left(element\right)\times number\ of\ atoms\ of\ element\ in\ the\ compound}{Mr\left(compound\right)}\right)$

  
  

  e.g. $m\left(C\right)\ =$$m\left(CO_2\right)\times\frac{12}{44}$
• equation to calculate the mass of an element in a compound (when you have the mols of the compound)

  
  

  $m\left(element\right)\ =$$\ n\left(compound\right)\ \times Ar\left(element\right)\ \times number\ of\ atoms\ of\ the\ element\ in\ the\ compound$

  
  

  e.g the mass of iodine in 0.062 mol of iodine monochloride =

  $0.062\ \times126.9\ =$$7.92287$
• Standard Titration Calculation Example:

  
• Back Titration Calculation Example 1 :

  
• Back Titration Calculation Example 2 :

  
• Gas Calculation Example:

  
• CORE PRACTICAL 1 - measuring the molar volume of a gas:

  (method 1 = displacement of water)

  Sources of error:

  • gas produced dissolving into your acid — add a pinch of your solid reactant to your acid prior to your experiment so some of your gas is produced, dissolves into the acid, and saturates the solution

  
  

  • gas produced dissolving into the water in the measuring cylinder as its collected — use a gas syringe.

  
  

  • gas produced being lost in the time it takes for you to replace the bung having added your solid reactant to your acid — place your solid reactant into a smaller container and place this inside of your acid containing conical flask. Replace the bung, and now, the reaction only takes place when the container is knocked over.
• Indicators: