C3

Question 1

What is the law of conservation for mass?

Answer 1

It states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants

Question 2

Because of the law of conservation for mass, what does it lead to?

Answer 2

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 3

Note:

Answer 3

AQA says that students should understand the use of the multipliers in equations in normal script before a formula and in subscript within a formula

(Check page 125)

Question 4

What is the relative formula mass of a compound?

Answer 4

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

Question 5

What is the sum of the relative formula masses equal to in a balanced chemical equation?

Answer 5

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

Question 6

Note:

Answer 6

AQA says that students should be able to calculate the percentage by mass in a compound given the relative formula mass and the relative atomic masses

Question 7

Why do some reactions involve a change in mass? (State an example too)

Answer 7

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

For example: 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 8

Note:

Answer 8

AQA says that students should be able to explain any observed changes in mass in non-enclosed systems during a chemical reaction given the balanced symbol equation for the reaction and explain these changes in terms of the particle model.

(Check CGP page 127)

Question 9

Whenever a measurement is made, what is there always?

Answer 9

Some uncertainty about the result obtained

Question 10

Note:

Answer 10

AQA says that students should be able to:

• represent the distribution of results and make estimations of uncertainty
• use the range of a set of measurements about the mean as a measure of uncertainty.

Question 11

What are chemical amounts measured in and what is its symbol?

Answer 11

• Chemical amounts are measured in moles
• The symbol for the unit mole is mol

Question 12

What is the mass of one mole of a substance in grams numerically equal to?

Answer 12

Its relative formula mass

Question 13

What does one mole of a substance contain?

Answer 13

The same number of the stated particles, atoms, molecules or ions as one mole of any other substance

Question 14

What is the number of atoms, molecules or ions in a mole of a given substance and what is its value?

Answer 14

• The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant
• The value of the Avogadro constant is 6.02 x 10^23 per mole

Question 15

What does the measurement of amounts of moles apply to and state an example

Answer 15

The measurement of amounts in moles can apply to atoms, molecules, ions, electrons, formulae and number equations

For example that in one mole of carbon (C) the number of atoms is the same as the number of molecules in one mole of carbon dioxide (CO2)

Question 16

State the equation to find the number of moles

Answer 16

Mass of g (Of an elements or compound)
Number of moles = ——————————————
Mr (Of the element or compound)

Question 17

Note:

Answer 17

Students should be able to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa.

(Do practice questions on moles with answers)
(Check CGP page 124)

Question 18

How do you calculate the masses of reactants and products?

Answer 18

Balanced symbol equations

Question 19

How can chemical equations be interpreted?

Answer 19

In terms of moles. For example:

Mg + 2HCI -> MgCl2+ H2

(This shows that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas)

Question 20

Note:

Answer 20

AQA says that students should be able to:

• calculate the masses of substances shown in a balanced symbol equation
• calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product.

Question 21

How can balancing numbers in a symbol equation be calculated?

Answer 21

The balancing numbers in a symbol equation can be calculated from the masses of reactants and products by converting the masses in grams to amounts in moles and converting the numbers of moles to simple whole number ratios

Question 22

Note:

Answer 22

AQA says that students should be able to:

• to balance an equation given the masses of reactants and products.
• change the subject of a mathematical equation.

(Do practice questions on these)

Question 23

What is common to use in a chemical reaction involving two reactants?

Answer 23

In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used

The reactant that is completely used up is called the limiting reactant because it limits the amount of products

Question 24

What is the effect of a limiting quantity of a reactants on the amount of products it is possible to obtain in terms of amounts in moles or masses in grams?

Answer 24

The mass of product formed in a reaction depends upon the mass of the limiting reactant. This is because no more product can form when the limiting reactant is all used up

Question 25

How is the concentration of a solution measured in?

Answer 25

Many chemical reactions take place in solutions. The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm^3 (g/dm^3).

Question 26

State the equation to find concentration

Answer 26

Concentration(g/dm^3) = Mass of sulte(g) / Volume of solute(dm^3)

Question 27

Explain how the mass of a solute and the volume of a solution is related to the concentration of the solution.

Answer 27

Concentration can be increased by dissolving more solute in a given volume of solution - this increases the mass of the solute. concentration can be increased by allowing some of the solvent to evaporate - this decreases the volume of the solution