Quantitative Chemistry

Question 1

Describe what is Meant by the Conservation of Mass

Answer 1

The law of conservation of mass states that no atoms are lost or made in a chemical reaction. Instead, the atoms join together in different ways to form products

Question 2

Explain why the Mass in a Reaction Appears to Change if a Reactant or Product is a Gas

Answer 2

-When gases in the air react, the products appear to gain mass as the mass of the gas was not included in the mass of the reactants at the beginning.

-When gases are produced in the reaction, the products appear to lose mass as the gas escapes to the atmosphere and its mass is not included

Question 3

Describe what is Meant by an Ion

Answer 3

An ion is an atom or molecule that has lost or gained electron(s) to form a stable electronic structure.

Question 4

State which Ions Metals and Non-metals Form

Answer 4

-Metallic elements lose electrons and form positive ions . The ion formed has a full outer shell so is stable.
e.g. Group 1- +1
Group 2- +2
Group 3- +3

-Non-metallic elements gain electrons and form negative ions. The ion formed has a full outer shell so is stable.
e.g. Group 7- -1
Group 6- -2

Question 5

Explain why it is Difficult to Determine the Charge on Transition Metal Ions

Answer 5

Transition metals can have a range of different positive charges so we cannot look at any transition metal and easily work out the charge it will have in a compound.

Question 6

Explain what (II) or (III) Mean After a Formula

Answer 6

-(III) means that the ion has a charge of 3+ in the compound and (II) means that the ion has a charge of 2+ in the compound.

-Transition metals often have small numbers after them. Because transition metals can have a range of charges, it’s usual for the formula of the compound to show the charge on the transition metal ion

Question 7

Describe what is Meant by Relative Formula Mass

Answer 7

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

Question 8

Describe how to Calculate the Relative Formula Mass of a Compound

Answer 8

-To find the relative formula mass of a compound, add together the atomic masses of each element in the compound.

-If an element appears more than once in the compound, multiply its mass by how many of its atoms are in the compound.

-If there is a big number in front of the formula, do not include that in the Mr of the compound.

Question 9

Describe what is Meant by the Range

Answer 9

-Range is the difference between the smallest value and the largest value.

Range = Largest Value - Smallest Value

Question 10

Describe what is Meant by the Mean

Answer 10

-Mean is the sum of all values divided by the number of values.

Mean = Sum of all Values / Number of Values

Question 11

Describe what is Meant by the Uncertainty

Answer 11

-Uncertainty is the mean plus or minus the range divided by 2

Uncertainty = mean +- range/2

Question 12

Give the Calculation for the Percentage Mass of an Element in a Compound

Answer 12

Percentage Mass of an Element in a Compound =

(Ar x Number of Atoms of that Compound) / Relative Formula Mass of the Compound

-x100 for the percentage

Question 13

State the Value of Avogadro’s Constant

Answer 13

-Avogadro’s constant is the number of atoms, molecules or ions in a mole of any substance.

Its value is:

6.02 x 10^23

Question 14

Give the Calculation for the Number of Moles in an Element

Answer 14

Number of Moles = Mass (g) / Relative Atomic Mass

n = m / Ar

Question 15

Give the Calculation for the Number of Moles in a Compound

Answer 15

Number of Moles = Mass (g) / Relative Formula Mass

n = m / Mr

Question 16

Explain what is Meant by the Limiting Reactant

Answer 16

-The limiting reactant is the reactant that is completely used up.

-The amount of limiting reactant determines the maximum amount of products that we can make.

Question 17

Explain how Limiting Reactant is Found

Answer 17

-Once the number of moles has been calculated, divide the number of moles by the ratio of the reactants.

-The reactant with smaller number of moles is limiting.

Question 18

Describe what is Meant by Concentration

Answer 18

-A solute is a chemical dissolved in a solvent. Water is often used as a solvent.

-The concentration tells us the mass of a solute in a given volume of solution.

Question 19

Give the Calculations for Concentration

Answer 19

Concentration (g/dm^3) = Mass of Solute (g) / Volume of Solvent (dm^3)

c = m/v

Concentration (mol/dm^3) = Number of Moles of Solute (mol) / Volume of Solvent (dm^3)

c = n/v

Question 20

Explain how the Mass of the Solute or Volume of the Solution can Affect Concentration

Answer 20

-If we increase the mass of solute, then the concentration increases.

-If we increase the volume, then the concentration decreases.

Question 21

Give the Calculation for Percentage Yield

Answer 21

Percentage Yield = Mass of Product Actually Made / Maximum Theoretical Mass of Product

-x100 for the percentage

Question 22

Explain why the Percentage Yield of Any Reaction Cannot be Greater then 100%

Answer 22

-The percentage yield for any reaction cannot be greater than 100% because this would mean that we had made atoms.

-The law of conservation of mass tells us that atoms cannot be lost or made during a chemical reaction.

Question 23

Give Reasons for why Percentage Yield is Always Less than 100%

Answer 23

-The reaction may not have gone to completion (reversible).

-Reactants may have been impure.

-Some reactants may have reacted differently to the expected reaction.

-We may lose some of the product when we separate it from the reaction mixture.

Question 24

Describe what is Meant by Atom Economy

Answer 24

The atom economy (which is also called atom utilisation) is a measure of the amount of starting materials that end up in useful products.

Question 25

Give the Calculation for Atom Economy

Answer 25

Atom Economy = Relative Formula Mass of Desired Products / Relative Formula Mass of All Reactants

-x100 for the percentage

Question 26

State One Environmental Advantage of Calculating Atom Economy

Answer 26

-Many reactants are produced with a negative impact on the
environment (for example mining can damage habitats, some reactants are produced using a lot of energy which could lead to climate change).

-If we can find a reaction with a greater atom economy then less of the atoms of reactants end up in waste products. This means that we can produce the same amount of product using less reactant.

Question 27

State One Economic Advantage of Calculating Atom Economy

Answer 27

-Reactants cost money to buy so if some of the atoms of our reactants end up in waste products, then we have wasted money.

-If we can find a reaction with a greater atom economy, we could save money as less atoms will end up in waste products.

-Atom economy is only one of the factors that influence the cost of a reaction (eg energy usage).

Question 28

Explain why we Cannot Have an Atom Economy Greater than 100%

Answer 28

-We cannot have an atom economy greater than 100% because that would mean that we had made atoms.

-The law of conservation of mass tells us that atoms cannot be lost or made during a chemical reaction.

Question 29

Describe what Scientists mean by Room Temperature and Pressure

Answer 29

Room temperature is 20°C and room pressure is 1 atmosphere.

Question 30

Give the Calculation for Volume of Gas

Answer 30

Volume of Gas (dm^3) = Moles x 24

v = 24n