Question

A mercury mirror forms inside a test tube as a result of the thermal decomposition of mercury(ii) oxide: 2 hgo(s) → 2 hg(l) + o2(g) δhrxn = 181.6 kj (a) how much heat is needed to decompose 996 g of the oxide?

Answer 1

The amount of heat needed to decompose 996 g of the mercury (ii) oxide is; 417.68 Kj

       We are given the balanced chemical equation of the thermal decomposition of mercury(ii) oxide as;

2HgO (s) → 2Hg (l) + O₂ (g)

     We want to find how much heat is needed to decompose 996 g of the oxide.

Thus;

Mass of HgO = 996 g

      We know from tables that molar mass of HgO is 216.59 g/mol

Thus,

number of moles of HgO = mass of HgO/Molar mass of HgO

number of moles of HgO = 996/216.59

number of moles of HgO = 4.6 moles

     From the question, we see that heat for the decomposition is;

δ_hrxn = 181.6 Kj

This means that;

2 moles of HgO (s) used 181.6 Kj of heat

Thus; 4.6 moles will use; 4.6 × 181.6/2 = 417.68 Kj

    In conclusion, the amount of heat needed to decompose 996 g of the mercury (ii) oxide is 417.68 Kj

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Answer 2

By definition, the amount of heat needed to decompose 996 g of HgO is 417.68 kJ.

The balanced chemical equation is:

2HgO (s) → 2Hg (l) + O₂ (g)

You want to find the amount of heat needed to decompose 996 g of the oxide.

So, first of all, it is convenient to express the mass of oxide in moles. For that, you know that molar mass of HgO (this is, the amount of mass a substance contains in one mole) is 216.59 g/mole

Then, the number of moles that contain 996 g of HgO can be calculated as:

$number of moles of HgO = \frac{mass of HgO}{Molar mass of HgO}$

$number of moles of HgO = \frac{996 grams}{216.59 \frac{grams}{mole} }$

number of moles of HgO = 4.6 moles

On the other side, you can see that heat for the decomposition is  181.6 kJ .

This means that  2 moles of HgO used 181.6 kJ of heat.

Then, the amount of heat used for 4.6 moles of HgO can be calculated with a role of three as follow: If 2 moles of HgO used 181.6 kJ of heat, 4.6 moles of HgO used how much heat?

$amount of heat used=\frac{4.6 molesx181.6 kJ}{2 moles}$

amount of heat used= 417.68 kJ

In conclusion, the amount of heat needed to decompose 996 g of HgO is 417.68 kJ.

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