1.2 Amount of substance Flashcards
What is relative atomic mass (Ar)
The average mass of an atom relative to 1/12th of the mass of an atom of carbon-12.
What is relative molecular mass (Mr)
- The average mass of a molecule relative to 1/12th of the mass of an atom of carbon-12.
- Sum of the relative atomic masses of each atom within the molecule.
What is avogadro’s constant (L)
L = 6.022x10²³
What is the formula for calculating the number of moles
- moles = mass/Mr
What is the formula for calculating concentration
- mass = concentration x volume
- moles = concentration x volume
What are 3 trends followed when gas and liquids are under standard conditions
- Pressure is proportional to temperature
- Volume is proportional to temperature
- Pressure is inversely proportional to volume
What is the ideal gas equation
pV = nRT
What are the units for each variable
- p = pressure in Pascals (Pa)
- V = volume in m³
- T = temperature in Kelvin (K)
- n = moles (mol)
- R = ideal gas constant (8.31 JK⁻¹mol⁻¹)
How do you convert from degrees celcius to kelvin
Add/ subtract 273
What is empirical formula
- The simplest whole number ratio of atoms of each element in a compound.
- It is found using molar ratios of each element.
What is molecular formula
- The true number of each atom in a molecule.
- It can be determined using the Mr of the empirical formula and the true Mr of the molecule.
- This gives a multiplier value which can be used to scale up the empirical formula.
- multiplier = Mr of molecule/ empirical Mr.
What is the equation for percentage yield
% yield = (actual mass/ theoretical mass) x 100
What is the equation for atom economy
atom economy = (Mr of desired product/ Mr of reactants) x 100
What is atom economy
- A measure of the efficiency of a chemical reaction.
- It considers the mass of all the atoms of reactants that end up as the desired product.
Why is calculating atom economy useful
It takes into account the atoms that end up in unwanted waste products as well as the yield of the reaction.
What is the difference between atom economy and yield
- Percentage yield: compares the amount of product produced to the amount that should’ve been produced.
- % Atom economy: Proportion of the mass of the products that is the desired product.
- Reactions can therefore have a high yield but a low atom economy if many useless by-products are produced.
Describe the method for preparing a volumetric/ standard solution
- Weigh out a dry sample of the substance in a weighing boat using a high precision balance (at least 2.d.p).
- Transfer to a beaker and reweigh the weighing boat to determine the mass transferred.
- Calculate the difference in mass.
- Dissolve the substance in a small volume of distilled water, stirring with a glass rod.
- Transfer to a volumetric flask using a funnel and rinse all equipment, transferring rinsings too.
- Add deionised water up to the calibration curve, using a dropping pipette at the end. Ensure the meniscus rests on the calibration line.
- Stopper the bottle and invert 10 times to homogenise the solution.
Describe the method for an acid-base titration
- Rinse the burette with the solution you will be filling it with to remove any contamination.
- Read the volume of the solution in the burette from the bottom of the meniscus at eye level (to avoid parralax error) and record the volume in a table.
- Rinse the volumetric pipette with the other solution and rinse the conical flask with distilled water.
- Use the volumetric pipette to transfer 25cm^3 of the other solution into the conical flask.
- Add 2 drops of a suitable indicator, e.g. phenolphthalein.
- Place a white tile under the conical flask to make the colour change more clear.
- Slowly add the solution in the burette, swirling between additions. Stop at the first permanent colour change.
- Repeat titration until concordant data is achieved and use these values to calculate a mean titre.
How do you decide which solution (acid or base) goes into the burette
- The solution in the burette is the solution that you know the concentration of.
- E.g. if the titration was between HCl and NaOH and you knew the concentration of NaOH, you would pour NaOH into the burette.
Why is distilled water used to rinse the conical flask
So there is no residue of chemicals in the flask that could affect the measurements.
What is concordant data
Data which is within 0.1 of each other, e.g. 25.20 and 25.30
How do you calculate the unknown concentration
- Calculate the moles of the solution in the burette added by using the equation n=cxv
- Find the ratio between the solutions and multiply/divide the calculated mole value accordingly.
- Calculate the concentration by using the formula c=n/v (volume is 25cm^3 or 0.025dm^3).
