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In carbon dioxide (CO₂), 12 g of carbon (C) combines with 32 g of oxygen (O). If 36 g of carbon is used, how much oxygen will combine according to the law of constant proportions?
96 g
64 g
48 g
72 g
96 g
Given: 12 g of Carbon (C) requires 32 g of Oxygen (O) to form CO₂. We have 36 g of Carbon.
12 g of Carbon (C) requires 32 g of Oxygen (O) to form CO₂. We have 36 g of Carbon.
m2m1=O2O1
Carbon increased from 12g to 36g, which is a factor of 3 (36 ÷ 12 = 3). Therefore, oxygen must also increase by a factor of 3: 32g × 3 = 96g.
Students often try to calculate the total mass of the compound first or miscalculate the ratio by adding instead of multiplying.
State the Law of Constant Proportions
According to the Law of Constant Proportions, chemical compounds always contain elements in a fixed ratio by mass. For CO2, the ratio of C:O is 12:32.
Ratio=3212
Set up the proportion
If 12g of C needs 32g of O, then for 36g of C, the required mass of oxygen x follows the ratio:
3212=x36
Calculate the unknown mass
Solve for x: x=1236×32. Since 36÷12=3, we have x=3×32=96g.
x=96g
A is correct because applying the constant mass ratio of 12:32 to 36g of carbon results in 96g of oxygen.
This concept is foundational for solving problems in Stoichiometry and Mole Concept (Chemistry), which appear frequently in RRB and SSC chemistry sections.