Calculations Involving Masses

Question 1

what is relative formula mass, Mr,

Answer 1

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

Question 2

what is avagadro constant

Answer 2

6.02 x10^23, whihc is the number of atoms, ions or molecules in one mole of a substance

Question 3

number of moles of a substance =

Answer 3

mass(g) / molar mass

Question 4

atoms =

Answer 4

moles x 6.02 x 10^23

Question 5

percentage of an element in a compound

Answer 5

(number of atoms of that element x Ar) / molar mass of the compound x 100

Question 6

volume of gases =

Answer 6

moles x 24

Question 7

1 dm^3 =

Answer 7

1000cm^3

Question 8

concentration of solution =

Answer 8

moles / volume

Question 9

concentration in g dm^3 =

Answer 9

concentration in mol dm^3 x molar mass

Question 10

concentration in g dm^3 =

Answer 10

mass of solute / volume

Question 11

what is empirical formula

Answer 11

the simplest whole number ratio of each element in a compound

Question 12

how to find empirical formula

Answer 12

1) mass / Ar

2) amount / smallest amount

Question 13

when does a reaction have a high atom economy

Answer 13

when more of the starting materials end up as useful products

Question 14

why is it important to use reactions with high atom economy

Answer 14

there is less waste
fewer reactants are used/needed
the process is more sustainable because it conserves natural resources

Question 15

atom economy =

Answer 15

Mr of useful product / sum of relative formula masses of all the reactants x 100

Question 16

how do you improve the atom economy of a reaction

Answer 16

find uses for the by products of the reaction

Question 17

percentage yield =

Answer 17

actual yield / theoretical yield x 100

Question 18

what are the reasons why the actual yield is less than the theoretical yield

Answer 18

1. some of the product is lost when it is seperated from the reaction mixture
2. there may be unwanted side reactions taking place which compete with the main reaction
3. the reaction may not go to completion possibly because the reaction has not been left for long enough or the reaction is reversible and reaches equilibrium

Question 19

why is excess reactant to used

Answer 19

to make sure all the acid completely reacts

Question 20

why is excess reactant to used

Answer 20

to make sure all the acid completely reacts