One gram of gold sample will contain 90 /100 = 0.9  gram of gold 

Number of moles of gold =mass of gold / atomic mass of gold

0.9 / 197 = 0.0046
One moles of gold contains NA atoms = 6.022 x 10²³
∴0.0046 mole of gold will contain = 0.0046  x 6.022 x 10²³
= 2.77 x 10²¹ 

 

Atoms of different elements join together in definite proportions to form molecules of compounds. Examples— water, ammonia, carbondioxide. Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion and the positively charged ion is called cation. Examples— sodium chloride, calcium oxide

Mass of 1 mole of aluminium atom = the molar mass of aluminium = 27 g mol^-1
 An aluminium atom needs to lose three electrons to become an ion, AI³⁺
 For one mole of AI³⁺ ion, three moles of electrons are to best lost. 
 
 The mass of three moles of electrons 3 x (9.1x10^-28) x 6.022 x10²³ g

 = 27.3 x 6.022 x10^-5 g
 
=164.400 x 10⁵ g
=.00164 g

Molar mass of Al3+ = (27 - 0.00164) gmol^-1
= 26.998gmol^-1

Difference = 27 – 26.9984 = 0.0016 g

(a) In a chemical reaction, the sum of the masses of the reactants and products remains
unchanged. This is called __________.
Ans. Law of conservation of mass
(b) A group of atoms carrying a fixed charge on them is called __________.
Ans. Polyatomic ion
(c) The formula unit mass of Ca₃(PO₄)₂ is __________.
Ans. (3 × atomic mass of Ca) + (2 × atomic mass of phosphorus) + (8 × atomic mass of oxygen) = 310
(d) Formula of sodium carbonate is and that of ammonium sulphate is __________.
Ans. Na₂ CO₃; (NH4)₂SO4

(a) Caustic potash
Ans. KOH
(39 + 16 + 1) = 56 g mol^-1
(b) Baking powder
Ans. NaHCO₃
23 + 1 + 12 + (3 × 16) = 84 g mol^-1
(c) Lime stone
Ans. CaCO₃
40 + 12 + (3 × 16) = 100 g mol^-1
(d) Caustic soda
Ans. NaOH
23 + 16 + 1 = 40 g mol^-1
(e) Ethanol
Ans. C₂H₅OH = C2H6O
2 × 12 + (6 × 1) + 16 = 46 g mol^-1
(f) Common salt
Ans. NaCI
23 + 35.5 = 58.5 g mol^-1