Chem All About Mole

What mass of oxygen would be needed to form
a) 2 moles of CO2

Mole ratio O2 : CO2 = 1 : 1
No. of moles of CO2 = 2 mol
No. of moles of O2 needed = 2 mol
Molar mass of O2 = 16*2 = 32 g/mol
Mass of O2 = 32 * 2 = 64 g

b) 4.6g of NO2

Mole ratio O2 : NO2 = 1 : 1
Molar mass of NO2 = 14 + 16x2 = 46 g/mol
No. of moles of NO2 = 4.6/46 = 0.1 mol
No. of O2 needed = 0.1 mol
Mass of O2 needed = 32 * 0.1 = 3.2 g

c) 6.02 x 10^23 formula units of MgO

Mole ratio O2 : MgO = 1 : 2
No. of moles of MgO = (6.02 * 10^23)/(6.02 * 10^23) = 1 mol
No. of moles of O2 = 1 * (1/2) = 0.5 mol
Mass of O2 needed = 0.5 * 32 = 16 g
(Not 1.6 g)


2.86g of hydrated salt ......

Mole ratio M2CO3.nH2O : H2O
2.86/286 : (2.86 - 1.06)/(1*2 + 16) = 1 : n
0.01 : 0.1 = 1 : n
0.01n = 0.1 * 1
Hence, n = 10

Let y be the atomic mass of M.
Formula mass of M2CO3•10H2O = 286
2y + 12 + 16*3 + 10(1*2 + 16) = 286
2y = 46
y = 23
Atomic mass of M = 23


19.85g of element M ......

Mole ratio M : O
= 19.85/31 : 25.61/16
= 0.640 : 1.600
= 2 : 5
Hence, simplest formula = M2O5


Yellow ammonium (NH4)2 Sx contains 72.72% by mass of Sulphur Find X

Mole ratio NH4 : S
(100 - 72.72)/(14 + 1x4) : 72.72/32.1 = 2 : x
1.516x = 2.265 * 2
x = 3


0.166 mole of a hydrated salt ......

Mole ratio (Hydrated salt) : H2O
= 0.166 : 17.94/(1*2 + 16)
= 1 : 6
No. of moles of water of crystallization in one mole of hydrated salt = 6


A hydrated salt contains ......

Let MX•10H2O be the hydrated salt,
and let y g/mol be the molar mass of the hydrated salt.

Consider 1 mole of MX•10H2O.
% by mass of water :
{[10 * (1*2 + 16)]/y} * 100% = (100 - 37.06)%
18000/y = 62.94
y = 286
Molar mass of the hydrated salt = 286 g/mol