quantitative chemistry

Question 1

law of conservation of mass

Answer 1

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 formula mass of a compound

Answer 2

the sum of the relative atomic masses of the atoms in the numbers shown in the formula

Question 3

what are the features of a balanced chemical equation

Answer 3

the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown

Question 4

when is the mass of a substance numerically equal to its relative formula mass

Answer 4

when you have one mole of a substance

Question 5

what does one mole of a substance contain in relation to other substances

Answer 5

it contains the same number of the states particles, atoms, molecules or ions as one mole of any other substance

Question 6

what are chemical amount measured in

Answer 6

moles (mol)

Question 7

what is Avogadro’s constant

Answer 7

the number of atoms, molecules or ions in one mole of a given substance is 6.02 x 10²³

Question 8

why should you use a reactant in excess in a chemical reaction involving two reactants

Answer 8

to ensure that all of the other reactant is used up

Question 9

define limiting reactant

Answer 9

the first reactant that is completely used up in a reaction; in a reaction, it is the reactant with the least number of moles

Question 10

when is the only case where you should include the big number in calculations

Answer 10

when calculating atom economy (you should multiply the big number by the Mr). this is the ONLY case where we do this

Question 11

what happens when the limiting reactant is all used up

Answer 11

no more of the product can form and the reaction stops

Question 12

define the reactant in excess

Answer 12

the reactant that is left over after the reaction stops; this happens because the limiting reactant has been used up

Question 13

define concentration in terms of volume

Answer 13

the amount of solute dissolved in a given volume (dm³) of solution

Question 14

why is it not always possible to achieve 100% yield

Answer 14

• the reaction may not go to completion because it is reversible
• some of the product may be lost when it is separated from the reaction mixture e.g. some left in the apparatus
• some of the reactants may react in ways different to the expected reaction
• the reaction may not have fully completed

Question 15

define atom economy

Answer 15

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

Question 16

what is high atom economy important for

Answer 16

it is important for sustainable development and for economic reasons to use reactions with high atom economy

Question 17

units of concentration

Answer 17

mol/dm³
OR
g/dm³

Question 18

units of volume

Answer 18

dm³

Question 19

units of moles

Answer 19

mol

Question 20

units of atom economy and percentage yield

Answer 20

%

Question 21

rule for equal amounts in moles of gases

Answer 21

equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure

Question 22

define molar gas volume

Answer 22

the volume occupied by one mole of any gas at room temperature and pressure; 24dm³