C3 - Quantitative Chemistry

Question 1

What does the law of conservation of mass state (2)

Answer 1

• That no atoms are lost or made during a chemical reaction so the mass of the product equals the mass of the reactants
• This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation

Question 2

Relative formula mass (Mr)

Answer 2

The relative formula mass of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula

Question 3

Relative formula masses of reactants in a balanced chemical equation

Answer 3

The quantities shown equals the sum of the relative formula masses of the products in the quantities shown

Question 4

Mases change when a reactant or product is a gas

Answer 4

Some reactions may appear to involve a change in mass but this can usually be explained because a reactant or a product is a gas and its mass has not been taken into account

Question 5

Mases change when a reactant or product is a gas - example (3)

Answer 5

• When a metal reacts with oxygen
• The mass of the oxide produced is greater than the mass of the metal or in thermal decompositions of metal carbonates
• Carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product

Question 6

Observed changes in mass in non-enclosed systems during a chemical reaction (2)

Answer 6

• No atoms are created or destroyed

* Instead they join together

Question 7

Moles (3)

Answer 7

• Is what chemical amounts are measured in
• Symbol unit is mol
• The mass of one mole of a substance is numerically equal to its relative formula mass - particles, atoms molecules or ions

Question 8

Avogadro constant (2)

Answer 8

• Is the number of atoms, molecules or ions in a given substance
• Has a value of 6.02 x 10(23) per mole

Question 9

Calculation for the number of moles

Answer 9

Number of moles = mass in g/relative formula mass (Mr)

Question 10

How to calculate the masses of substances shown in a balanced symbol equation

Answer 10

Mass = number of moles x relative formula mass

Question 11

How to calculate the balancing numbers in a symbol equation (2)

Answer 11

Can be calculated from the masses of reactants and products:
• By converting the masses into grams to amounts of moles and converting the number of moles to simple whole number ratios

Question 12

How to balance an equation given the masses of reactants and products (4)

Answer 12

• Divide the mass of each substance by its relative formula mass to find the number of moles
• Divide the number of moles of each substance by the smallest number of moles in the reaction
• If any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers
• Write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas

Question 13

Limiting reactant

Answer 13

Is the reactant that is completely used up therefore limits the amount of products

Question 14

Explain the effect of a limiting quantity of a reactant on the amount of products (2)

Answer 14

• The reaction stops when all of one of the reactants is used up - all other are in excess
• The amount of product formed is directly proportional to the amount of limiting reactant

Question 15

What is the concentration of a solution measured in

Answer 15

Grams per dm(3)

g/dm(3)

Question 16

How to calculate the mass of solute in a given volume of solution of known concentration in terms of mass per given volume of solution

Answer 16

Mass = concentration x volume 
g = g/dm(3) x dm(3)

Question 17

How is the mass of a solute and the volume of a solution related to the concentration of the solution (2)

Answer 17

• solute of a solution is directly proportional to the concentration of a solution in a given volume
• As the volume increases the concentration decreases (inversely proportional)

Question 18

How to calculate relative atomic mass?

Answer 18

Sum of (isotope abundance x isotope mass number)/ sum of abundances of all the isotopes