Chapter 4: Chemical Calculations

Question 1

Relative masses and moles

Answer 1

• The masses of atoms are compared by measuring the rfm to atoms of carbo-12
• You can add up the RFMs of all the elements of a compound to find the total rfm
• Number of moles = mass(G) / rfm
• One mole of any substance is its rfm in grams
• Avogadro constant is 6.02 x 10^23 per mole

Question 2

Equations and calculation

Answer 2

• Balanced symbol equations tell you the number of moles of substances involved in a chemical reaction
• You can use balanced symbol equations to calculate the masses of reactants and products in a chemical reaction

Question 3

From masses to balanced equations

Answer 3

• You can deduce balanced symbol equations from the masses of substances involved in a chemical reaction
• The reactant that gets used up first in a reaction is called the limiting reactant. This is the reactant that is not in excess
• Therefore, the amounts of product formed in a chemical reaction are determined by the limiting reactant

Question 4

The yield of a chemical reaction

Answer 4

• Percentage yield = (mass of product produce / maximum mass of product possible) x 100
• Factors affecting the yield of a chemical reaction include product left behind in the apparatus, reversible reactions not completing, some reactants may produce unexpected reactions, and losses in separating the products from the reaction mixture.

Question 5

Atom economy

Answer 5

• Atom economy = (rfm of desired product/ total rfm of products) x 100
• It is important to maximise atom economy in the industrial process to conserve the earth’s materials and minimise pollution

Question 6

Expressing concentrations

Answer 6

• Concentration (g/dm3) = amount of solute (g) / volume of solution (dm3)
• dm3 = cm3 / 1000

Question 7

Titrations

Answer 7

• titration is used to measure accurately what volume of acids is needed to neutralise a certain volume of alkali
• the acid is in the burette, measure the amount, making sure there are no bubbles
• you use the pipet to measure the volume of alkali
• concordant results mean results which are with 0.1cm3 of each other

Question 8

Titration calculations

Answer 8

• concentration (mol/dm3) = number of moles (mol) / volume (dm3)
• the balanced equation gives you the number of moles in a certain volume of solution, once you know the accurate concentration

Question 9

Volume of gases

Answer 9

• a certain volume of gas always contains the same amount of molecules under the same conditions
• the volume of 1 mole of any gas is 24dm3 (24 000 cm3)
• you can use the molar gas volume and balanced equations to calculate the volumes of gaseous reactants or products