Question

When 5 g of calcium is burnt in 2 g of oxygen, then 7 g of calcium oxide is produced. What mass of calcium oxide will be produced when 5 g of calcium is burnt in 20 g of oxygen? Which law of chemical combination will govern your answer?

Answer 1

$2\mathrm{Ca}+{\mathrm{O}}_2\to 2\mathrm{CaO}$
From the equation, the number of moles of CaO formed is equal to the number of moles of calcium used.
So, from the question,
Mass of calcium = 5 g
Mass of oxygen = 2 g
Mass of calcium oxide = 7 g
Number of moles of oxygen = $\frac{1}{2}$ Number of moles of calcium
Moles of calcium = $\frac{5}{40}$ = 0.125
Moles of oxygen = $\frac{2}{32}$ = 0.0625
So, the number of moles of calcium is greater than the number of moles of oxygen.
Hence, the oxygen atom is the limiting reagent.

Therefore, the number of moles of calcium oxide = number of moles of oxygen.
Now, when 5 g of calcium is used in 20 g of oxygen, then
The number of moles of oxygen = $\frac{20}{32}=0.625$
The number of moles of calcium = $\frac{5}{40}$ = 0.125
Here, the number of moles of calcium is lesser than the number of moles of oxygen.
So, here calcium is the limiting reagent.
Number of moles of calcium oxide = $\frac{7}{56}=0.125$
The number of moles of calcium oxide = Number of moles of calcium.
So, the weight of calcium oxide formed in the second case is 7 g.

Law of conservation of mass is applicable to this equation.

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