We are given a chemical reaction scenario involving calcium (Ca) and zinc carbonate (ZnCO₃), producing calcium carbonate (CaCO₃) and zinc (Zn). The question asks:
How much zinc was present in the zinc carbonate, based on the
law of conservation of mass.
Let’s analyze the information:
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Given:
- Calcium (Ca): 64 g
- Zinc carbonate (ZnCO₃): 192 g
- Calcium carbonate (CaCO₃): 152 g
- Zinc (Zn): ? g (we need to find this)
The reaction is:
> Ca + ZnCO₃ → CaCO₃ + Zn
This is a single displacement reaction where calcium displaces zinc from zinc carbonate.
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Step 1: Apply the Law of Conservation of Mass
The law states that
mass is neither created nor destroyed in a chemical reaction. So:
> Total mass of reactants = Total mass of products
So:
> Mass of Ca + Mass of ZnCO₃ = Mass of CaCO₃ + Mass of Zn
Plug in known values:
> 64 g + 192 g = 152 g + Mass of Zn
> 256 g = 152 g + Mass of Zn
Now solve for Mass of Zn:
> Mass of Zn = 256 g - 152 g =
104 g
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Step 2: Interpret the Question
The question asks:
"How much zinc was present in the zinc carbonate?"
We just calculated that
104 g of zinc was produced. Since zinc came only from the zinc carbonate (ZnCO₃), and no other source of zinc is involved, the amount of zinc
in the original zinc carbonate must be
104 g.
Even though the total mass of ZnCO₃ was 192 g, that includes both zinc and carbonate. But we now know that
104 g of that was zinc, and the rest (192 - 104 = 88 g) was carbonate (CO₃²⁻).
But the question specifically asks:
how much zinc was present in the zinc carbonate?
✔ Answer:
104 g
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Final Answer:
C) 104 g
✔️ Correct choice:
C
Parent Tip: Review the logic above to help your child master the concept of law of conservation of mass example problems.
