Quantitative chemistry

Question 1

How do you calculate the relative formula mass of a substance

Answer 1

The relative atomic masses of all the atoms in the molecular formula added together.

Question 2

State the value of the Avogadro constant

Answer 2

6.02 x 10^23

Question 3

What is the formula that relates the number of moles of a substance to its mass and relative formula mass

Answer 3

Number of moles=Mass/Relative formula mass

Question 4

What does conservation of mass mean

Answer 4

During a chemical reaction no atoms are destroyed and no atoms are created

Question 5

Suggest why the mass of a reaction vessel might decrease during a reaction

Answer 5

Because one of the products is a gas and all reactants are solids, liquids or aqueous. The gas can escape from the reaction vessel so you can’t account for it’s mass

Question 6

How can you determine the number of moles of each substance that would react together from the balanced reaction equation

Answer 6

The big numbers in front of the chemical formulas of the reactants and products tell you how many moles of each substance takes part or are formed

Question 7

Explain what is meant by the term limiting reactant

Answer 7

The reactant that is used up in a reaction as it limits the amount of product formed

Question 8

What volume does one mole of any gas occupy at room temperature and pressure

Answer 8

24dm^3

Question 9

State the values of room temperature and pressure

Answer 9

20C 1atm

Question 10

What is concentration

Answer 10

The amount of substance in a certain volume of a solution

Question 11

Give the equation for working out the concentration of a solution in g/dm^3

Answer 11

concentration=mass of solute/mass of solvent

Question 12

Give the equation for working out the concentration of a solution in mol/dm^3

Answer 12

concentration=number of moles of solute/volume of solvent

Question 13

Describe how you would convert a concentration in mol/dm^3 to g/dm^3

Answer 13

Use the equation mass=moles x Mr

Multiply the concentration in moles by the relative formula mass

Question 14

Give the equation for calculating the atom economy of a reaction

Answer 14

Atom economy= (Relative formula mass of desired products/relative formula mass of all reactants) x 100

Question 15

Give three reasons why it’s better to use reactions that have a high atom economy

Answer 15

• Atoms with a low atom economy produce a lot of waste that is expensive to dispose
• Atoms with a low atom economy use up resources very quickly
• A lot of waste produced will harm the environment

Question 16

What’s the atom economy of a reaction that only produces one product

Answer 16

100%

Question 17

What information do you need to find the theoretical yield of a reaction

Answer 17

The balanced symbol equation

Question 18

Give the equation for calculating the percentage yield of a reaction

Answer 18

Percentage yield= mass of product actually made/maximum theoretical mass of product

Question 19

Give two reasons why it’s better to use reactions with high percentage yields

Answer 19

• To reduce costs

- To reduce waste

Question 20

Give three reasons why percentage yield is always less than 100%

Answer 20

• Not all reactants react to make a product, in reversible reactions the products can turn back into reactants
• There might be side reactions, some reactants might react with gases in the air or impurities in the reaction mixture forming extra products
• You lose some product when you separate it from the reaction mixture, when filtering or transferring containers some particles are left behind

Question 21

What is the atom economy of a reaction

Answer 21

The atom economy of a reaction tells you how much of the mass of the reactants is wasted when manufacturing a chemical and how much ends up as useful products