Test: Atoms And Molecules - 1 - UPSC

Methane (CH₄) is a gas and we know for gases at STP,
22.4 L = 1 mole gas
⇒ 4.48 L of methane gas = 4.48/22.4 = 0.2 moles
∵ 1 mole of CH₄ = 6.022 x 10²³ molecules of methane
∴ 0.2 mole of CH₄ = 6.022 x 10²³ x 0.2 = 1.2 x 10²³ molecules

The law of definite proportion states that a compound always contains exactly the same proportion of elements by mass. Thus, no matter where the carbon dioxide comes from, it always carbon and oxygen in the ratio by mass.

In 12 g of carbon, the amount of C-14
= 12 × 2/100 = 0⋅24g
∴ C-14 atoms in 0.24g = (0⋅24×6⋅02×10²³)/14
= 1⋅03×10²² atoms.

• One mole of CO₂ = one mole of C = 6.02×10²³ atoms of carbon.
• One mole of CO₂ = two moles of O = 2×6.02×10²³= 12.04×10²³ atoms of oxygen.
• One mole of CO₂​ = 6.02×10²³ molecules of CO₂

​Note: 6.02×10²³ is Avogadro's number and represents number of atoms/molecules present in one mole of substance.

The chemical symbol for gold is Au, taken from the Latin word for gold- Aurum. Its atomic number is 79.

There is also one mole of oxygen atoms weighing 16.00 grams in the mole of water and that of hydrogen is 2.00 gram.
To get the percentage of hydrogen, divide the 2 by 18 and multiply by 100.
i.e. Percentage of hydrogen = (2/18) x 100 = 11.11%

• The combining capacity of an element is known as its valency.
• Valency is the number of valence electrons in outer shell that take part in the chemical reaction.

One atomic mass unit is defined as one-twelfth of the one-carbon of one carbon - 12 atom because C - 12 is chosen as the standard form.

1 gram - atom nitrogen corresponds to mass of one mole of the nitrogen atom is grams that is one gram - atom of nitrogen represents 28 gms of nitrogen. 
It is equal to 6.023 * 10²³ N atoms, but not  
6.023 * 10²³ N2 molecules.
 

Avogadro's number, number of unit in one mole of any substance equal to 6.023 * 10²³ . The units may be electrons, atoms, ions or molecules, depending on the nature of the substance and the character of the reaction.