1.2 Amount of substance

Question 1

What is relative atomic mass (Ar)

Answer 1

The average mass of an atom relative to 1/12th of the mass of an atom of carbon-12.

Question 2

What is relative molecular mass (Mr)

Answer 2

• 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.

Question 3

What is avogadro’s constant (L)

Answer 3

L = 6.022x10²³

Question 4

What is the formula for calculating the number of moles

Answer 4

• moles = mass/Mr

Question 5

What is the formula for calculating concentration

Answer 5

• mass = concentration x volume
• moles = concentration x volume

Question 6

What are 3 trends followed when gas and liquids are under standard conditions

Answer 6

• Pressure is proportional to temperature
• Volume is proportional to temperature
• Pressure is inversely proportional to volume

Question 7

What is the ideal gas equation

Answer 7

pV = nRT

Question 8

What are the units for each variable

Answer 8

• 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)

Question 9

How do you convert from degrees celcius to kelvin

Answer 9

Add/ subtract 273

Question 10

What is empirical formula

Answer 10

• The simplest whole number ratio of atoms of each element in a compound.
• It is found using molar ratios of each element.

Question 11

What is molecular formula

Answer 11

• 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.

Question 12

What is the equation for percentage yield

Answer 12

% yield = (actual mass/ theoretical mass) x 100

Question 13

What is the equation for atom economy

Answer 13

atom economy = (Mr of desired product/ Mr of reactants) x 100

Question 14

What is atom economy

Answer 14

• 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.

Question 15

Why is calculating atom economy useful

Answer 15

It takes into account the atoms that end up in unwanted waste products as well as the yield of the reaction.

Question 16

What is the difference between atom economy and yield

Answer 16

• 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.

Question 17

Describe the method for preparing a volumetric/ standard solution

Answer 17

• 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.

Question 18

Describe the method for an acid-base titration

Answer 18

• 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.

Question 19

How do you decide which solution (acid or base) goes into the burette

Answer 19

• 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.

Question 20

Why is distilled water used to rinse the conical flask

Answer 20

So there is no residue of chemicals in the flask that could affect the measurements.

Question 21

What is concordant data

Answer 21

Data which is within 0.1 of each other, e.g. 25.20 and 25.30

Question 22

How do you calculate the unknown concentration

Answer 22

• 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).