C2: The Mole Concept

Question 1

definition of relative atomic mass

Answer 1

the average mass of one atom of an element compared to one-twelfth the mass of an atom of carbon-12

Question 2

why is relative atomic mass used instead of actual masses when measuring atoms

Answer 2

because atoms are too small to be measured directly.

Question 3

what isotope of carbon is used as the standard for determining relative atomic mass (A_r)?

Answer 3

Carbon-12

Question 4

what is the unit of A_r ?

Answer 4

relative atomic mass has no units

Question 5

why relative atomic mass has no units

Answer 5

relative atomic mass is a comparison of masses with that of carbon-12

Question 6

what is the unit mass of each atom of carbon-12?

Answer 6

12 amu (atomic mass units)

Question 7

Since one carbon atom has a mass of 12 amu, then the mass of 1/12th the mass of carbon 12 is ______

Answer 7

1 amu

Question 8

the masses of ionic and covalent compounds are compared to cabon 12 atoms also and give rise to the term (1)______________

and (2)______________

Answer 8

1. relative formula mass
2. relative molecular mass

Question 9

definition of relative molecular mass

Answer 9

the average mass of one molecule of an element or compound compared to 1/12th the mass of an atom of carbon 12.

Question 10

relative molecular mass and relative formula mass are calculated by ________________

Answer 10

adding together all the relative atomic masses of all the elements present in the compound

Question 11

definition of relative formula mass

Answer 11

the average mass of one molecule of an ionic compound compared to 1/12th the mass of an atom of carbon 12.

Question 12

which numbers represent the relative atomic mass

Answer 12

1, 2, 3 ,6 , 7, 8

Question 13

definition of a mole

Answer 13

the amount of a substance that contains 6x10²³ particles of the same substance

Question 14

what is Avogadros constant?

Answer 14

6 x 10²³

Question 15

hwo to calculate relative molecular and relative formula mass

Answer 15

add all the A_r of all the elements in the compound

Question 16

how would you define 1 mole of any gas

Answer 16

the volume of gas that contains 6x10²³ molecules of the gas

Question 17

(1)The ___________________ of a substance is the mass of 1 mol of the substance and is equal to the (2)________________, (3)_________________, or (4)_______________ expressed in grams

Answer 17

1. molar mass
2. relative atomic mass,
3. relative molecular mass,
4. formula mass

Question 18

the molar mass of an element or compound is simply its (1) ______________, (2)______________ or its (3)_______________ expressed in grams

Answer 18

1. relative atomic mas
2. relative molecular mass
3. relative formula mass

Question 19

when performing mole calculations involving mass of a substance apply this formula triangle

Answer 19

Question 20

when performing mole calculations involving number of particles apply this formula triangle

Answer 20

Question 21

when performing calculations involving volume of gases apply this formula triangle

Answer 21

Question 22

molar volume is defined as (1)_________________ and 1 mol of a gas containes (2)__________ molecules,

therefore (3)____________ of a gas contains 6x10²³ molecules. The molar volume of any gas is (4)__________ at stp and (5) ___________ at rtp.

Answer 22

1. the volume occupied by 1 mole of a gas
2. 6x 10²³
3. 1 mole
4. 22.4dm³
5. 24.0dm³

Question 23

the molar volume of any gas contains this amount of molecules

Answer 23

6 x 10²³

Question 24

THE MOLE AS THE ‘MIDDLE MAN’

what is common with all these triangle formluas and what does this mean:

N_A - Avogadros constant

N - number of particles

M_r - Relative molecular or relative formula mass

V_m - Molar volume

V - volume

C- concentration

n - number of moles

m - mass

Answer 24

All the formulas n - (number of moles). This means that the number of moles will either be given or needs to be calculated in order to complete the triangle calulations

Question 25

what do chemical formulas represent?

Answer 25

how many of moles of each element are combined to form 1 mole of a compound

Question 26

what do you understand from the following formulae: 1. CO₂ 2. NH₄ 3. CaCO₃ 4. H₂SO₄

Answer 26

1. 1 mol of carbon combines with 2 mol oxygen to form 1 mol carbon dioxide 2. 1 mol of nitrogen combines with 4 mol hygrogen to form 1 mol of ammonia 3. I mol calcium combines with 1 mol carbon and 3 moles oxygen to form 1 mole calcium carbonate 4. 2 moles hydrogen combines with 1 mol sulphur and 4 moles oxygen to form 1 mol sulphuric acid

Question 27

this give the simplest mole ratio between atoms or ions present in a compound

Answer 27

empirical formula

Question 28

this gives the actual number of moles of atoms of each element in one mole of a compound

Answer 28

molecular formula

Question 29

Ionic compounds are represented by (1)\_\_\_\_\_\_\_\_\_\_\_, while covalent compounds are represented by (2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Answer 29

1. empirical formula 2. molecular formula

Question 30

what is the ratio of the atoms in the following compounds 1. CO₂ 2. NH₄ 3. CaCO₃ 4. H₂SO4

Answer 30

1. 1:2 2. 1:4 3. 1:1:3 4. 2:1:4

Question 31

definition of empirical formla

Answer 31

the simples mole ratio of each atom or ion present in a compound

Question 32

molecular formula definition

Answer 32

the actual number of moles of _atoms of each element_ present in 1mol of a compound

Question 33

difference between relative formula mass and relative molecular mass

Answer 33

relative formula mass is used when refering to ionic compounds while relative molecular mass refers to covalently bonded elements or compounds

Question 34

can one determine the molecular formula of a salt?

Answer 34

no. salts are ionic compounds and so are _always_ represented by empirical formulas

Question 35

How to determine the empirical formula of an ionic compound in which the ratios of its elements are expressed in percentages Hint: draw a table

Answer 35

1. write down the relative atomic mass of each of the elements in the compund 2. give a percentage value to all of the elements in the compound. The sum of all the elements represents 100%, so missing percentages are calculated using simple algebra 3. let 100% = 100g and convert each percentage value to grams by simply changing their units form % to grams. 4. convert each given mass to moles 5. divide each result by the smallest mole amount. This will give the simplest mole ratio which is the empirical formula.

Question 36

How to determine the molecular formula of a compound with known relative molecular mass and in which the ratios of its elements are expressed in percentages. Hint: draw a table

Answer 36

1. write down the relative atomic mass of each of the elements in the compound 2. give a percentage value to all of the elements in the compound. The sum of all the percentages is 100%, so missing percentages are calculated using simple algebra 3. convert each percentage value to grams by simply changing their units form % to grams. 4. convert each given mass to moles 5. divide each result by the smallest mole amount. This will give the simplest mole ratio of the elements, which is the empirical formula. 6. determine the relative molecular mass of the emiprical formula. we now have 2 relative molecular masses - one for the actual compound and one for the empirical formula. 7. Determine the ratio between the 2 relative molecular masses. This must be a whole number equal to or greater than one. 8. multiply the empirical formula by this value to obtain the molecular formula of the compound.

Question 37

How to determine the molecular formula of a compound with known relative molecular mass, in which the masses of its elements are known Hint: draw a table

Answer 37

1. write down the relative atomic mass of each of the elements in the compound and determine the molar mass of each element 2. convert each given mass to number of moles 3. divide each result by the smallest mole amount. This will give the simplest mole ratio of the elements, which is the empirical formula. 4. determine the relative molecular mass of the emiprical formula. we now have 2 relative molecular masses - one for the actual compound and one for the empirical formula. 5. Determine the ratio between the 2 relative molecular masses. This must be a whole number equal to or greater than one. 6. multiply the empirical formula by this value to obtain the molecular formula of the compound.

Question 38

How to determine the empirical formula of an ionic compound in which the masses of its elements are known Hint: draw a table

Answer 38

1. write down the relative atomic mass of each element in the compound and determine its molar mass 2. convert each given mass to moles 3. divide each result by the smallest mole amount. This will give the simplest mole ratio which is the empirical formula.

Question 39

important points to consider when given problems to determine the empirical and molecular formulas of compounds.

Answer 39

* only ionic compounds are represented by empirical formula * molecular compounds are represened by molecular formulas * The actual *M_r* of the compound is divided by the *M_r* of the empirical formula to give a whole number ratio * if the problem gives the percentages of each element, convert each percentage to grams by simply replacing the % with g. Then convert to number of moles