(1) Here you first calculate the amount of substance from the mass and the molar mass. Then you multiply by the constant.
first: n = m / M then: n * 6 * 10²³ = number of atoms
a) n = 4 g / 24 g/mol number of atoms = 0,166 mol * 6 * 10²³ 1/mol
n = 0,166 mol number of atoms = 0,96 * 10²³
b) n = 0,12 g / 12 g/mol number of atoms = 0,01 mol * 6 * 10²³ 1/mol
n = 0,01 mol number of atoms = 0,06 * 10²³
c) n = 32 g /48 g/mol number of molecules = 0,666 mol * 6 * 10²³ 1/mol
n = 0,666 mol number of molecules = 4 * 10²³
d) n = 14,2 g / 71 g/mol number of molecules = 0,2 mol * 6 * 10²³ 1/mol
n = 0,2 mol number of molecules = 1,2 * 10²³
Sodium sulfate has the formula Na2SO4, so it has two sodium atoms per
molecule. So you have to multiply the number of molecules by 2.
number of atoms = number of molecules * 2
number of atoms = 1,2 * 10²³ * 2
number of atoms = 2,4 * 10²³
(2) The amount of substance can easily be calculated:
n = m / M
a) n = 16 g / 16 g/mol
n = 1 mol
b) n = 0,14 g / 14 g/mol
n = 0,01 mol
c) n = 5,4 g / 108 g/mol
n = 0,05 mol
d) n = 8,88 g / 35,5 g/mol
n = 0,25 mol
(3) With the same formula the mass can now be calculated, for this it only has to be converted according to the mass:
n = m / M | *M
m = n * M
a) m = 0,05 mol * 16 g/mol
m = 0,8 g
b) m = 10 mol * 23 g/mol
m = 230 g
c) m = 0,01 mol * 1 g/mol
m = 0,01 g
d) m = 0,25 mol * 65,4 g/mol
m = 16,35 g
(4) The molar mass of a molecule is obtained by adding the molar masses of the individual atoms:
a) Br = 80 g/mol → Br2 = 2 * 80 g/mol = 160 g/mol
b) C = 12 g/mol, O = 16 g/mol → CO2 = 12 g/mol + 2 * 16 g/mol = 44 g/mol
c) H = 1 g/mol, N = 14 g/mol, O = 16 g/mol
→ HNO3 = 1 g/mol + 14 g/mol + 3* 16 g/mol = 63 g/mol
d) Cu = 64 g/mol, S = 32 g/mol, O = 16 g/mol, H = 1 g/mol
→CuSO4*5H2O = 64 g/mol + 32 g/mol + 4 * 16 g/mol + 5 * (2 * 1 g/mol +
16 g/mol) = 250 g/mol
(5) Exercise 5 works exactly like exercise 2 and 4:
first: Calculate molar mass then: n = m / M
a) n = 128 g / 16 g/mol
n = 8 mol
b) M = 39 g/mol + 14 g/mol + 3 * 16 g/mol
M = 101 g/mol
n = 25,25 g / 101 g/mol
n = 0,25 mol
c) M = 2 * 12 g/mol + 6 * 1 g/mol + 16 g/mol
M = 46 g/mol
n = 414 g / 46 g/mol
n = 9 mol
(6) Exercise 6 works exactly the same as exercises 3 and 4:
first: Calculate molar mass then: m = n * M
a) M = 32 g/mol + 2 * 16 g/mol
M = 64 g/mol
m = 2 mol * 64 g/mol
m = 128 g
b) M = 2 * 1 g/mol + 32 g/mol + 4 * 16 g/mol
M = 98 g/mol
m = 20 mol * 98 g/mol
m = 1960 g
c) M = 23 g/mol + 16 g/mol + 1 g/mol
M = 40 g/mol
m = 0,5 mol * 40 g/mol
m = 20 g