topic 3 - quantitative chemistry

Question 1

what does the law of conservation of mass state?

Answer 1

the law of conservation of mass 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

what are ions?

Answer 2

atoms with a charge

Question 3

how are positive and negative ions formed?

Answer 3

• metals form positive ions and non-metals forms negative ions

Question 4

what are positive non-metal ions?

Answer 4

• hydrogen ion
• ammonium ion

Question 5

what is the rule for ionic compounds?

Answer 5

• in an ionic compound, the charges on the ions have to cancel out to leave an overall charge of zero

Question 6

how do you calculate the %mass of an element in a compound?

Answer 6

percentage mass of an element in a compound = Ar x number of atoms of that element / Mr of the compound
x 100

Question 7

how do you calculate the number of moles?

Answer 7

mass (g) / Mr

Question 8

what is Avogadro’s constant?

Answer 8

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

Question 9

Calculate the number of atoms in 28g of lithium. Ar Li = 7

Answer 9

number of moles = 4 moles
number of atoms = 4 x 6.02 x 10^23
number of atoms = 2.408 x 10^24

Question 10

what does a limiting reactant in a chemical reaction mean?

Answer 10

• 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 11

what is meant by concentration?

Answer 11

• concentration tells us the mass of a solute in a given volume of solution

Question 12

what is a solute?

Answer 12

a solute is a chemical that is dissolved in a solvent

Question 13

what is often used as a solvent in chemistry?

Answer 13

water

Question 14

what is the unit for concentration?

Answer 14

one unit of concentration is g/dm^3

Question 15

what is the equation for concentration?

Answer 15

concentration (g/dm^3) = mass (g) / volume (dm^3)

Question 16

what happens if we increase the mass of a solute?

Answer 16

• if we increase the mass of a solute and keep the volume the same, then we increase the concentration

Question 17

what happens if we increase the volume of the solution?

Answer 17

• if we increase the volume of solution and keep the mass of the solute the same, then we decrease the concentration

Question 18

why is it not always possible to achieve 100% yield in a chemical reaction?

Answer 18

• some of the product may be lost when it is separated from the reaction mixture
• some of the reactants may react in different ways to the expected reaction so we do not get the product we expect
• reversible reactions may not go to completion

Question 19

what is the equation for percentage yield?

Answer 19

percentage yield = actual yield / theoretical yield x 100

Question 20

why do industries aim for a high percentage yield?

Answer 20

• lower costs and less waste

Question 21

what is atom economy?

Answer 21

atom economy is a measure of the amount of starting materials that end up as useful products

Question 22

what are two reasons why it is important to have a high atom economy?

Answer 22

• less waste is produced
• more sustainable as less raw materials are used up

Question 23

what is the equation to calculate atom economy?

Answer 23

atom economy = Mr of desired product / total Mr of reactants x100

Question 24

what is meant by concentration? (T.S.)

Answer 24

• concentration tells us the number of moles of a solute in a given volume of solution

Question 25

what is the unit of concentration? (T.S.)

Answer 25

• one unit of concentration is mol/dm^3

Question 26

what is the equation linking concentration, moles and volume?

Answer 26

concentration = number of moles / volume

Question 27

what is the volume of one mole of any gas at room temperature and pressure?

Answer 27

24dm^3

Question 28

what is room temperature?

Answer 28

20°C

Question 29

what is room pressure?

Answer 29

1 atmosphere

Question 30

what is the equation to calculate the volume of a gas?

Answer 30

volume (dm^3) = number of moles x 24