1.2 Amounts of Substance

Question 1

Define relative atomic mass:

Answer 1

• the weighted average mass of all the isotopes, relative to 1/12th of the mass of 1 atom of carbon-12

Question 2

Define relative molecular mass:

Answer 2

• the mass of a molecule of a compound, relative to 1/12th of the mass of an atom of carbon-12

Question 3

Define mole:

Answer 3

• the amount of substance that contains as many particles as there are in exactly 12g of carbon-12

Question 4

Give the equation to find number of atoms:

Answer 4

• Number of atoms = number of moles x Avogadro’s constant

Question 5

What are the assumptions made when using the ideal gas equation?

Answer 5

• there are no intermolecular forces between the molecules
• the volume occupied by the molecules themselves is entirely negligible relative to the volume of the container

Question 6

Define empirical formula:

Answer 6

• the simplest whole number ratio of atoms of each element present in a compound

Question 7

Define molecular formula:

Answer 7

• the actual number of atoms of each element present in a compound

Question 8

Give the equation for percentage error:

Answer 8

% error = (uncertainty / value) x 100

Question 9

What are the steps taken to calculate water of crystallisation?

Answer 9

• calculate masses of the hydrated and anhydrous salts
• mass of H2O = mass of hydrated salt - mass of anhydrous salt
• calculate moles of anhydrous salt and water
• find the simplest whole number ratio

Question 10

Define theoretical yield:

Answer 10

• the number moles expected if the reactants were completely converted into products

Question 11

Define actual yield:

Answer 11

• the actual number of moles of product formed in a reaction

Question 12

What are 4 possible reasons for low percentage yield?

Answer 12

• incomplete reaction/ reversible reaction
• unwanted side reactions
• loss of product
• impurities in the reactants

Question 13

Give the equation for atom economy:

Answer 13

• Atom economy = (mr of desired product / total mr of all products) x 100

Question 14

Describe the method used to make a standard solution:

Answer 14

• put the beaker on the balance and set it to 0
• add the solid to the beaker and record its mass
• dissolve the solid in distilled/deionised water
• transfer solution into a volumetric flask using a funnel
• make up to the graduated line by slowly adding deionised water using a pipette (meniscus sits on the line)
• stopper the flask and invert to mix the contents thoroughly

Question 15

Describe the method to carry out an accurate titration:

Answer 15

• fill a burette with a standard solution of the acid
• pipette a known volume of base into a conical flask
• add a few drops of indicator to the flask
• slowly add the solution of acid from the burette to the base until the indicator changes colour (end point)
• record the volume of acid added
• perform a rough titration followed by accurate ones until you get concordant results (+- 0.1cm3)

calculate the mean titre from the concordant results