Test: Atoms And Molecules - 1


10 Questions MCQ Test Science & Technology for UPSC CSE | Test: Atoms And Molecules - 1


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QUESTION: 1

4.48 L of CH4 at STP is equal to______.

Solution:

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

QUESTION: 2

The same proportion of carbon and oxygen in CO2 obtained from different sources proves the law of_______.

Solution:

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.

QUESTION: 3

If isotopic distribution of C-12 and C-14 are 98% and 2% respectively, then number of C-14 atoms in 12 g of carbon is________.

Solution:

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.

QUESTION: 4

1.0 mole CO2 contains

Solution:
  • 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.

QUESTION: 5

Identify the correct symbol of Gold. 

Solution:

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

QUESTION: 6

The percentage of hydrogen in water is______.

Solution:

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%

QUESTION: 7

The combining capacity of an element is called 

Solution:
  • 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.
QUESTION: 8

One atomic mass unit of carbon-12 is

Solution:

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.

QUESTION: 9

Which is not represented by 1 mole of nitrogen gas? 

Solution:

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.
 

QUESTION: 10

Numerical value of Avogadro’s number is

Solution:

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.

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