Answers to Review notes:
1- n=m/Mmass
n=4.2g/(23+1+12+(3x16)
n= 4.2g/84g/mol
n=0.05mol
2- n=m/Mmass
0.75mol=g/((12x8)+(1x18))
0.75ml=g/114g/mol
g=0.75molx114g/mol
g=85.5g
3- The number of atoms is given by Avogadro’s number. To determine the number of
atoms in a sample we only need the number of moles, we do not need to know what
the substance is.
a. So the number of atoms in 1 mole of a substance is:
atoms
b) In 2 moles of calcium there are:
atoms
c) In 5 moles of phosphorus there are:
atoms
d) Before we can work out the number of atoms, we must work out the number
of moles. The relative atomic mass of magnesium is 24.31 u. So we have 1 mole
of magnesium. (Relative atomic mass = sample mass)
The number of atoms is:
atoms
e) The relative atomic mass of carbon is 12.01 u
The number of moles in the sample is 2. (Relative atomic mass = half sample
mass). The number of atoms is:
atoms
4- For hydrogen we have 1.01 g of sample and 1.01 u for relative atomic mass, so we
have one mole of hydrogen. For carbon we have 24.0 g of sample and 12.0 u for
relative atomic mass, so we have two moles of substance (
)
Doing this for the rest of the elements in the table we get:
Relative atomic mass (u) Sample mass (g) Number of moles in the sample
Element
Hydrogen 1.01
101
1
Magnesium 24.3
24.3
1
Carbon
12.0
24.0
2
Chlorine
35.45
70.9
2
Nitrogen 14.0
42.0
3
5- If we look at the periodic table, we see that the molar mass of iron is 55.8 g/mol.
This means that 1 mole of iron will have a mass of 55.8 g.
If 1 mole of iron has a mass of 55.8 g, then: the number of moles of iron in 11.7 g
must be:
There are 0.2 moles of iron in the sample.
6- Molar mass of zinc is 65.4 g/mol, meaning that 1 mole of zinc has a mass of 65.4 g.
If 1 mole of zinc has a mass of 65.4g, then 5 moles of zinc has a mass of:
(answer to a)
7- a)
b)
c)
x2= 2.02g/mol
x2= 28.02g/mol
x2= 159.8g/mol
8- a) Molar mass of boron is:
If one mole of boron has a mass of 10 g then the number of moles of boron in 21,62
g must be:
b) Molar mass of manganese is:
If one mole of manganese has a mass of 59,908 g then the number of moles of
manganese in 54,94 g must be:
c) Molar mass of mercury is:
If one mole of mercury has a mass of 200,59 g then the number of moles of
manganese in 100,03 g must be:
d) Molar mass of barium is:
If one mole of barium has a mass of 137,33 g then the number of moles of barium in
50 g must be:
e) Molar mass of lead is:
If one mole of lead has a mass of 207,2 g then the number of moles of lead in 40 g
must be:
9-
There are 2 moles of copper in the sample.
10Number of atoms in 3 mol aluminium
There are 1,8069 × 1024 aluminium atoms in a sample of 81 g.
11- a) Molar mass of calcium:
b) Molar mass of manganese:
c) Molar mass of aluminium:
12- a) Molar mass of lead:
b)
13- a) Molar mass of magnesium:
b) Molar mass of lithium:
c) Number of mols of silicon
Silicon has a molar mass of
. The mass of the sample is:
14- Molar masses of each element:
Hydrogen = 1,01 g·mol−1
Sulphur = 32,1 g·mol−1
Oxygen = 16,0 g·mol−1
15- Convert mass into grams
Calculate the molar mass
Find the number of moles
There are 10,5 moles of magnesium chloride in a 1 kg sample.
16- The units of the mole : mol
17- a) Molar mass of
is:
b) Molar mass of
:
c) We divide by Avogadro's number to get the number of moles:
18- a)
b) There are 3 mol of calcium atoms. The number of calcium atoms is:
19- a) Molar mass of
is:
Mass is:
b) Molar mass of
Mass:
is: