Quantitative Chemistry

Question 1

Relative Formula Mass

Answer 1

Relative atomic masses of all the atoms in the molecular formula added together

Question 2

%age mass of an element in a compound

Answer 2

%age mass = Ar x number of atoms of that element / Mr of the compound (x100)

Question 3

Avagadro constant

Answer 3

6.02 x 10^23

602 000 000 000 000 000 000 000

Question 4

Moles

Answer 4

• avagadro number of atoms

- one mole is exactly the same number of grams as the relative atomic mass

Question 5

Equation linking moles, mass and Mr

Answer 5

Moles = mass / Mr

Mass = moles x Mr

Mr = mass / moles

Question 6

Conservation of mass

Answer 6

• same number of atoms (moles) on either side of the equation
• no atoms are created or destroyed
• total Mr of reactants = total Mr of reactants

Question 7

Why might the mass appear to change in a chemical reaction?

Answer 7

If one of the products or reactants is a gas, it won’t be contained in the reaction vessel, so you can’t account for its mass

Question 8

Limiting Reactant

Answer 8

• the reactant that is used up
• often the other reactants will be added in excess to ensure it is all used up
• amount of product formed is directly proportional to the limiting reactant; if you halve the limiting reactant, you will also halve the products

Question 9

At room temperature and pressure, one mole of any gas occupies…

Answer 9

24dm^3

Question 10

Concentration

Answer 10

• substances that are dissolved in a solution

- concentration: amount of a substance in a certain volume of a solution

Question 11

Equation for Concentration (using mass)

Answer 11

Concentration = mass of solute / volume of solvent

Question 12

Equation for Concentration (using moles)

Answer 12

Concentration = number of moles of a solute / volume of solvent

Question 13

Atom economy

Answer 13

• reactions produce useful and waste products
• atom economy tells you how much of the mass of the reactants ends up as the desired product
• 100% atom economy means all the atoms in the reactants have been turned into the desired product
• higher atom economy: ‘greener’ process

Question 14

Equation for atom economy

Answer 14

Atom economy = Mr of desired product / Mr of all reactants (x100)

Question 15

Percentage Yield

Answer 15

• compares actual and theoretical yield
• always somewhere between 0% and 100%
• 100% yield means you got all the product you expected to get and 0% means you got none
• industrial processes aim to have a high percentage yield to reduce waste and reduce costs

Question 16

Equation for %age yield

Answer 16

%age yield = mass of product actually made / maximum theoretical mass of product (x100)

Question 17

Can yields be 100%?

Answer 17

• yields of a product are always less than 100%

- some product/reactant always lost along the way

Question 18

Why might a percentage yield be lower than 100%?

Answer 18

• not all reactants react to make a product
• there might be side reactions
• you lose some product when you separate it from the reaction mixture

Question 19

State the values of room temperature and pressure

Answer 19

20 C and 1 atm

Question 20

Give three reasons why it’s better to use a reaction with a high atom economy

Answer 20

• less material needed to create desired product
• less waste produced
• lower costs for more product

Question 21

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

Answer 21

100% (all the atoms have gone to the same product)

Question 22

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

Answer 22

Formula masses and a balanced symbol equation

Question 23

Give two reasons why it’s better to have a high percentage yield

Answer 23

• lower costs

- reduces waste