Topic 3: quantitative chemistry

Question 1

what is the law of conversation of mass

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

define relative atomic mass

Answer 2

average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C

Question 3

define relative formula mass

Answer 3

sum of relative atomic mass of all atoms in the formula

Question 4

what is avogradro’s constant

Answer 4

the number of atoms, molecules, or ions in a mole of a given substance

Question 5

what is the formula that links mass, molecular mass, and moles together

Answer 5

mass = mr x moles

Question 6

state what we mean by a limiting reactant in a chemical reactions

Answer 6

in a chemical reaction involving 2 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 its limits the amount of products

Question 7

write down the 2 formulae that link concentration, mole/mass, and volume together

Answer 7

concentration = mass / volume
concentration = nr of moles / volume

Question 8

what is the molar volume of a gas at room temp and pressure

Answer 8

1 mole of a gas at room temp and pressure occupies 24 dm3

Question 9

what is titration

Answer 9

a technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration

Question 10

how do you conduct titration

Answer 10

1. rinse the pipette with a solution of unknown concentration. use the pipette to measure out the known volume of this solution
2. add an indicator
3. rinse the burette with a solution of known concentration. discard the liquid. use a burette to gradually add the solution of a known concentration
4. when indicator changes colour, the volume added is recorded
5. it is important to get concordant volume results, they have to lie close to each other
6. suitable calculations are performed to find the concentration

Question 11

why is it not possible to obtain the theoretical amount of production in a chemical reaction

Answer 11

• the reaction may not go completion because it is reversible
• some of the product may be lost when it is separated from the reaction mixture
• some of the reactants may react in ways different to the expected reaction

Question 12

how is the percentage yield of a product in a chemical reaction calculated

Answer 12

% yield = actual mass of a product/ maximum theoretical mass of product x 100

Question 13

what is atom economy

Answer 13

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

it is the ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants