Question

. 24 g of each of carbon and oxygen react to form CO2. The amount of un-reacted
element left behind is
A) 15 g
B) 6 g
C) 16 g
D) 10 g​

Answer 1

Answer:

1 mole of carbon = 1 mole of CO2

12g of carbon = 44g CO2

1 mole of O2 = 1 mole of CO2

Molecular mass of O = 16

Molecular mass of O2 = 32

Therefore, 32g of O2 = 44g of CO2.

This means 16 g of elements have not reacted.

Answer 2

Concept:

• No of moles = Given Mass/ Molecular Mass
• The ratio of the components of a compound is always fixed.

Given:
24g of C and 24 g of O2.

Find:

Amount of Unreacted element.

Solution:

1 mole combines with 1 mole of O2 to give 1 Mole of CO2.

No of moles of C in 24 gm = 24/12 =2 moles

No of moles of O2 in 24 g = 24/32 = 0.75 moles

Since the number of moles of O2 is only 0.75 so it can react with only 0.75 moles of C, and the rest of the C will remain unreacted.

Mass of 0.75 moles of C = 12* 0.75 = 9g

Unreacted Element left behind = 24-9 =15g

Only 0.75 moles of the C could combine with 24g oxygen so 1.25 mole of the C was wasted. The weight of 1.25 mole of C is 15g.

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